Human perception is a virtual reality in the sense that our perceptual environment is generated for us by the unconscious part of our brain. The brain bases this virtual reality on the processing of incoming neural signals. Those signals in turn are processed forms of the information coming into our senses from the outside world. In other words, we never perceive reality directly but instead perceive the inputs produced by our own virtual reality machine. That said, for most senses there is a clear correspondence between what we perceive and what's out there. Tones of higher and lower pitch correspond to air vibrations of higher and lower frequencies. A skin sensation tells us about an object touching our body at the same location as where we feel it. The patterns and distances we see correspond to the 3D locations of objects in our surrounding space. Although colors are also a consistent projection of real spectral properties of light reflections, the particular way our brain color codes the objects we see is very much an artistic fantasy of our visual system. Because of this I would say that colors might be classified as the most virtual part of our virtual reality machine.

The title of this article suggests that colorblindness is a myth. I do not mean that in the trivial sense that, though the term suggests otherwise, most people said to be colorblind do see everything in color rather than in black and white. I mean it's a myth that what distinguishes color deficient people from people with normal color vision is that the former cannot see the difference between certain spectral combinations. People with normal color vision also cannot see the difference between certain spectral combinations, so that is not a distinguishing characteristic. And, perhaps surprisingly, color deficient people can distinguish some colors which are undistinguishable by people with normal color vision. What characterizes color deficient people is that the variation of colors they see is smaller than that of people with normal color vision. This article discusses various aspects of color vision perception woven together by its theme: the myth of colorblindness.

Basic color perception theory

Although many people know that colors are related to wavelength, few people seem aware that what we call color is not a fundamental aspect of reality, but rather a description of the particular way our visual system works. There are two ways one could define color strictly in terms of physics. One way is to say that a color is light of a single wavelength. Another is to say that a color is a light spectrum, consisting of a combination of many different wavelengths in different intensities. What we call color is neither of these two things.

Color is not equal to single wavelengths, since that would only include the colors of the rainbow: violet, blue, green, yellow, orange and red (sometimes indigo is added between violet and blue and cyan between blue and green). But we see many more colors which do not exist in the rainbow, such as brown, pink, olive, black, grey and white. These non-rainbow colors are "fantasy colors" created by our visual system and arise from certain combinations of wavelengths.

Color is also not equal to a light spectrum, since we have only a very poor ability to discriminate different spectra. What we call a single color can be created by many different spectra. A spectrum is an infinite dimensional space, while color is only a three dimensional space. This is due to the fact that our color vision system works with three different types of light detectors (called "cones") each of which gives a single signal based on its wavelength sensitivity curve:

Here is a mathematical transform of the above curves used used in the 1931 CIE color model:

The curves in the first figure are normalized so that each has a peak of 1. Note the large degree of overlap between the curves. The rods, which are not show in the above figure are most sensitive to green and are primarily active during night vision. They produce a black and white image sensation. The curves in the second curve are based on imaginary primaries with negative light components each of which would stimulate only a single cone. This transformation is performed for purposes of linearity. For more information on this see Out of Gamut: Why Is Color.

One type of cone is primarily sensitive to short wavelengths (blue), another to medium wavelengths (green) and one to long wavelengths (yellow). The yellow cone is usually referred to as the red cone. While its sensitivity peak lies in the yellow wavelength band, it is also quite sensitive to red. The center of vision, the fovea, contains no rods and no blue cones. A single cone cannot detect color, as it provides only a scalar number indicating the total light energy it absorbs. For example, the red cone by itself cannot distinguish red from yellow, green or orange. Red is detected by a combination of high activation of the red cone, low activation of the green cone and no activation of the blue cone. That is why a color monitor can generate most colors by a combination of red, green and blue light. All humans can see in terms of color is a relative activation strength of its three types of light sensitive cones. This means that many complex spectra of light can only be differentiated by a spectrometer, such as a prism, while we cannot distinguish them with the eye. For example, a human sees no difference between a flat spectrum (such as sun light) and a graphic on a computer monitor whose spectrum consists of three spikes in the colors blue, green and red. Both are called white. Similarly, we cannot distinguish spectral yellow from a particularly balanced combination of red and green light. Additive color mixing, such as with a computer monitor, is something different than subtractive color mixing, as with paints. A computer monitor makes yellow by adding green and red light. A painter gets green, for example, by mixing cyan (blue-green) with yellow, because the cyan absorbs the red part of the spectrum, while the yellow absorbs the blue part of the spectrum, leaving green when both are absorbed by a mixture of both paints.

You might wonder how a computer monitor can create violet even though violet is on the wrong side of the color spectrum compared to blue. One can image how a computer can create cyan (blue-green) by adding green and blue, but how can it use regular blue to create violet, a bluer type of blue if we are to believe the rainbow? The answer lies in a sensitivity inversion of the color cones. The green cone sensitivity curve drops faster than the red cone sensitivity curve near the blue part of the spectrum. That is why people with normal color vision can hardly distinguish spectral violet from bluish-purple made by combining blue with a bit of red. And so a computer screen can simulate spectral violet with red and blue. If one does not take this into account in the photographic process, one runs the risk that spectral violet looks blue in a picture while a violet made from a reddish blue will be reproduced correctly.

An essential property of color vision is that unlike sounds we can't see two colors at the same time. Or in a sense we can, but we are not aware of it because our perception is always that of a single color. We can see green and red light at the same time, but we see it as a single color: yellow. We do not perceive blue-green as two simultaneous presences of blue and green, but see a single blue-green color. We cannot derive directly from our perception that yellow can be made by a combination of red and green. We only know this from systematic color vision testing. On the other hand we may suspect from our perception that blue-green is a combination of blue and green.

Colorblindness

The term colorblindness suggests that "colorblind" individuals cannot see certain colors. A more accurate description would be that they cannot distinguish certain colors. All "colorblind" individuals see every color, except for some people who cannot see red in the sense that red appears black to them. Blindness only refers to the fact that two colors can look the same which appear as different colors to a person with normal color vision. For more information about color vision see, for example, http://webvision.med.utah.edu/Color.html.

I have a form of "colorblindness" myself. I am a deuteranomalous trichromat - my green cone wavelength sensitivity curve is shifted toward my red cone curve. I didn't know I was "colorblind" until I failed a color vision test at age 18. On most of those dot images shown to me I saw nothing where I was supposed to see a number. Now these tests are very carefully designed so that "colorblind" individuals cannot see the color contrast. Under normal circumstances other people will typically not note my "colorblindness", since I can usually name most colors correctly. But with knowledge about color vision and "colorblindness" you'd be able to find my weak points. For example, you can confuse me with unusual light conditions. For one thing, my ability to distinguish certain colors seems to deteriorate much sooner than for other people with low light levels. And being green weak I may confuse a bright green light with a white light. One reason for this is presumably that a light bulb creates a loss of the option of comparing the brightness of the light to that of other objects. Under uniform light green is less bright than white and hence green and white are easy to distinguish. Another reason is that a bright green light is probably not pure green, but an unsaturated green (green mixed with white). A third reason is that a point light source covers a smaller retinal area with fewer cones able to participate in color discrimination. Similarly, with a low light level, I might see light grey as light green. Why should it not be the other way around? Why should I not see a bright white light as green and a light green surface as light grey? I think the reason for this is that light green surfaces are more prevalent than light grey surfaces. So I have learned that something light green or grey is usually green. Similarly, bright white lights are more common than bright green lights, so I have learned that a bright green or white light is usually white. I recently learned that the lighted EXIT letters in cinemas are green and not white, something I had always suspected but wasn't sure about and didn't care about. The EXIT letters now look greener to me than they used to. Another example of a mistake is that I might confuse a very dark pure green with black, and vice versa. And of course there's less distinction for me between all the colors in the red-green area such as red, green, brown, orange. Yellow tends to be very clearly different, probably because it is a brighter color. But I think I also see less difference than other people between certain shades of green and blue.

Fortunately I see a big difference between a green and red traffic light, partly because they have been kind enough to add some blue to the green for the color challenged and partly because traffic lights are not points but discs covering a larger area. Also, I can distinguish the green traffic light from white. If I had to tell red or green only from the top or bottom position it would have been quite difficult at night. But although I can normally distinguish orange and red, as with green and white, bright lights make this more difficult. So I hardly see any difference between the European orange and red traffic lights. The United States is friendlier because their middle light is yellow rather than orange, which is easier. At night I hardly see any difference between red traffic lights and neon orange street lights. But that's not too bad, because from position and context I can tell which is which far enough in advance.

When I learned I was "colorblind" I was a bit surprised. But the surprise was lessened by the fact that I remembered my mother had told me I might be colorblind a few times, since she had noticed I sometimes made mistakes naming colors. I never believed her, since I assumed from the word colorblindness that it meant viewing in black and white, while I saw everything in color. I did believe the doctor who gave me the test. I was not disappointed with this news since I had always been quite happy with the world which looked so colorful to me and I was only amazed to find out that most people saw a world which was even more colorful. However, is that always correct? Our artificial western world of color paintings, color TV, color websites, color magazines, colored book covers, colored furniture, colored cars, etc., might be more colorful for me than the world of an Eskimo, who sees nothing by white, a desert Nomad who sees only yellowish sand, and an amazon Indian who sees only green plants, even if they all have normal color vision.

The following table shows the prevalence of different types of inherited color vision:

My apologies that the figures for females don't add up to 100% - I copied these figures from "Clinical Procedures in Optometry" published by J.B. Lippincott (Chapter 13 by James E. Bailey). Inherited blue-yellow defects are very rare, but they do occur more frequently as acquired color deficiencies in old age, due to illness (e.g. diabetes) or from certain medicines. As can be seen from the table the largest groups with color deficiency are 6% of caucasian males with anomalous trichromatic vision and 2% of caucasian males with dichromatic vision. People with dichromatic vision suffer from the more severe form of "colorblindness", since they only have two cone types, while anomalous trichromats have all three cones, but the red and green cone sensitivity curves overlap more than is normal. Anomalous trichromatism exists in various degrees of severity, from borderline to mild, moderate and severe. Many are unaware of their defect. While many anomalous trichromats can correctly name most colors under most circumstances, dichromats have a very limited ability to distinguish colors in the red-green range such as green, red, brown and orange. On the other hand, their ability to distinguish blue, purple and yellow, for example, would probably be almost as good as that of a normal color vision individual. Both groups involve a defect of the red-green color system. People with protanopia cover a limited light spectrum and see deep red as black. Protanomalous people have this characteristic to a limited extent.

An interesting difference between normal color vision and color deficiency is how colors can mix. For example, like for most red-green defective people the color red-green makes some sense to me. I sometimes perceive what looks sort of like a mixed color between red and green. This makes no sense to other people since for them there is no mixed color between red and green because when you mix red and green you get an entirely different color: olive (or yellow with a light mix).

The contrast between yellow and red or between yellow and green is larger to me than the contrast between red and green. And I suspect most people with normal color vision will subjectively feel it to be the other way around. The two most contrasting colors for red-green deficient people are yellow and blue. But like most anomalous trichromatic red/green weak individuals, I can usually distinguish red and green quite well. The grass still looks green to me and a sunset beautifully red, something which cannot be said by protan or deutan dichromats. But with some shades of red and green and/or under bad lighting I might have trouble. However, if you show me red and green together under bad conditions my ability to tell which is which greatly improves, because now I can compare them.

What is particularly striking to me is that in almost all information about "colorblindness" there seems to be consensus both from normal color vision individuals and from "colorblind" individuals that "colorblind" individuals have some kind of colorblindness in that they cannot distinguish certain colors, while normal individuals supposedly have "full color" vision. This is an extremely misleading way to put it. According to this definition colorblindness is the disability to distinguish certain colors. But that is true only if we uphold normal color vision individuals as some kind of standard. Trichromats (3 types of color cones) are arbitrarily defined as having full color vision while dichromats (2 types of color cones) and anomalous trichromats (one defective and two normal types of color cones) are defined as having some kind of "colorblindness". But from an objective point of view the term "colorblindness" would make sense only in two ways: (1) as a description of achromatopsia (black and white vision) or (2) as the inability to distinguish certain light spectra. Since color weak individuals are diagnosed as having a form of colorblindness, apparently colorblindness is defined according to (2).

That said, I have some shocking news. Everybody is colorblind. That's right, since trichromatic vision provides only a very limited ability to distinguish color spectra, everybody is colorblind. Many color spectra can be made that are very different physically but which would look exactly the same to you, even if you have "full color vision". For ages normal color vision individuals and color deficient individuals have conspired to make 8% of the male population and 0.4% of the female population believe that they have a defect such that they cannot see certain colors while all other people can supposedly can see all colors. This is a myth. It is true that "colorblind" individuals have a less rich color vision system than normal individuals. But this is not a case of full color vision versus defective color vision. It is only a case of different degrees of defective color vision. A tetrachromatic (four types of color vision cones) creature may call a normal trichromatic human colorblind, just as a normal trichromatic human might call a dichromatic individual colorblind. For every pseudoisochromatic Ishihara test used to diagnose "colorblindness" I can make 1000 Ishihara tests with which normal color vision people can be diagnosed with "colorblindness", based on the fact that they cannot see the numbers printed even though a prism can clearly show different light spectra.

Invisible color coding

These ideas could be put to use to create invisible color coding. I've heard that some poker cheaters have used this idea to read marked cards using filter contact lenses. Let me give you an example of how this would work. The color yellow can be created as a single wavelength of 580 nm. Alternatively it can be created as a combination of red light and green light, as on a computer monitor. Now let's say I make two types of (dark) yellow paint, paint A made with pure yellow wavelength and paint B made from green plus red wavelength. I paint a piece of paper with paint A. On that I write a secret message with paint B. Nobody can see it, as it is simply yellow paint on yellow paint. I look at the paper through a simple red filter. I can now clearly see the message as red on black. What use does this invention have? I'm not sure. Perhaps as a spying game for children. Or to create codes on objects for logistics purposes while being invisible without filter glasses for esthetic purposes. Or, as mentioned, to make invisible markings on playing cards for purposes of cheating.

ColorMax lenses

Does this idea mean one can improve color vision with filters, by making otherwise invisible color differences visible? Not really. It makes some colors distinguishable at the price of making others indistinguishable. ColorMax offers filter lenses which are claimed to improve color vision for red-green color deficient individuals. But when I asked for information they sent me a report on a clinical study showing significantly improved results of color deficient individuals in the Farnsworth D-15 color vision test! This is obviously silly since the purpose of this test is to diagnose various color deficiencies and all the glasses do is invalidate the test. Improvement on the broader Farnsworth D-100 color vision test would have been more impressive. Being able to pass a color vision test with filter glasses is no proof of improved color vision. The real world is not made out of color vision test colors. Such proof would have to consist of a demonstration that the subject's total color space has increased.

In one sense it can be proved that filter lenses decrease color vision rather than improve it. All filter glasses can do is attenuate certain wavelength energies. But then every color I can see through the glasses is also a color I can see without the glasses. If an object is seen with color X through the glasses then I can create the same color sensation without the glasses by repainting the object with a darker paint equal to the original color modified by a wavelength attenuation curve corresponding to that of the glasses. On the other hand I cannot see every color with the glasses that I can see without the glasses. An example is white. The filter glasses can never simulate an object which reflects all light, since the filter will always make it darker and I cannot repaint an object with a color brighter than white. So without the glasses I can see all colors that are visible with the glasses, but not vice versa. Ergo, the number of colors I can see without the glasses is larger than the number of colors I can see with the glasses.

However, via the same proof it could be shown that if I reduce lighting or wear flat filter sunglasses I also see fewer colors. But one generally does not assume that a reduction of light decreases color space, except perhaps when it becomes relatively dark. For one thing, when it becomes darker my pupils widen and my eyes become more sensitive, compensating for the loss of light. For this and other reasons color space is usually defined as a two dimensional space, even though three color cone signals imply three dimensional space. Since the 19th century colors are often classified according to hue (dominant spectral wavelength), saturation (purity) and value (brightness). An unsaturated color is a mixture of that color with white. Pink, for example, is an unsaturated red. According to one estimate humans can distinguish about 200 hues, 20 levels of saturation and 500 steps of brightness. This gives a total of 2 million color variations. The ability to distinguish color variations in an absolute sense (naming a single color rather than seeing a difference between two colors), will of course be much less. Hue and saturation can be combined to give a two dimensional color space with a constant brightness, as in the standard CIE color model:

Pure spectral colors and the line of purples constitute the border of this color space. Note that many colors are missing here because only bright colors are shown. All other colors are darker versions of the colors shown. Brown, for example, is in fact dark orange. And olive is dark yellow. Part of our color perception is created by brightness comparison with surrounding colors. Here is a different version of the CIE diagram:

The spectral wavelengths are harder to read here, but the theory behind the chart can be better explained because the X and Y axes are numerated. The chart represents an XY plane projection of a three dimensional XYZ color space. X, Y and Z are akin to relative stimulation of respectively the red, green and blue cones. They are based on a mathematical transformation of the cone sensitivity curves as shown in one of the figures presented earlier. The 3 values are normalized so that they always add up to 1. For example, the point where red and green are both 0.4 represent a blue value of 0.2. The chart also demonstrates the inversion where X drops to 0 near spectral green-blue and then increases again at blue. (Actual red cone stimulation doesn't drop to 0 near spectral green-blue. Remember X, Y and Z are mathematical transformations of red, green and blue cone stimulation, but that falls outside of the scope of this article.)

I will now show that in this model ColorMax lenses neither increase nor decrease color perception. What the lenses do, presumably, is alter the relative stimulation of the 3 cones (or 2 cones) of color deficient people, by attenuating certain frequencies. Now the obvious question this raises is why they don't use this principle to improve the color vision of people with normal color vision. But let's proceed.

Let's take the case of the most common group, anomalous trichromats, whose red and green color curves more closely overlap than those of normal individuals. They will have a similar color space as the above diagram, except it will be crushed from the lower right and top left. Color space becomes smaller.

Now let's consider the color space as altered by the filter lenses (given any type of normal of defective color vision). Our first job is to show that every point in this space is also present in the unaltered space. This is easy. The same argument applies as above. Any object I see with the glasses can be simulated by taking my glasses off and recoloring the object with darker paint such that the same spectral attenuation occurs relative to the original paint. Now the other part. Can all the colors in the unaltered color space be seen in the space altered with filter lenses? Let's take an example. Suppose the lenses filter away part of the yellow light. We already know that the type of color space we are considering is invariant under brightness changes. So we can compensate the lack of yellow caused by the filter glasses by painting all objects in our vision field darker except for their yellow component. Then if I put the glasses on I see the same colors as before (without the glasses and without the repainting) except the brightness has uniformly decreased. So all colors in the unaltered color space are also present in the filtered color space. Hence, in this model, whatever type of filter we use (except a filter that filters out certain wavelengths completely) the colors we can see remain exactly the same.

ColorMax lenses might still be of use in that they might enable better perception of color coding schemes used by people with normal color vision at the expense of diminished color discrimination for other colors. So although the lenses don't change the colors we are able to see, perhaps they make our color discrimination closer to that of normal color vision individuals, for example by increasing the difference between red and green while decreasing the difference between blue and yellow. However, it must be emphasized that in our color space red, green, blue and yellow remain completely unchanged. The only things that the glasses change are our color labeling system and the particular colors objects happen to have. For example, the grass that used to be green might look brown now but what appears brown is now called green. And brown might look green but we call it brown. Very confusing. Our lifelong training in naming colors is thrown out the window and we can start all over learning a new system. And I happen to like the fact that grass looks green, sunsets look red and skies look blue and changing those colors doesn't strike me as esthetically pleasing. But we might have the illusion that we can see colors better, because when we've relearned the new system we might make fewer color naming errors in terms of the vocabulary of our super color companions. In particular it does not seem appealing to me that the most common color, white, will seem less pure with filter glasses. In fact the glasses delete pure white from our color space in the 3 dimensional color model, even though the 2 dimensional color model says white is equivalent to grey. But perhaps you get used to this and start to see white as pure white again after a while. Just like my white looks like pure white, even though compared to others the white I see is green-lacking, but I'm used to that since birth.

Another downside is that any filter lenses decrease brightness. Particularly inside that would tend to decrease real color discrimination, because while in one theory color space remains unchanged, in practice color discrimination does decrease with low levels of brightness. But the best argument against ColorMax is simply that if they really believed it worked they would make color improvement lenses for people with normal color vision as well.

I wonder, by the way, whether a two dimensional color model based on hue and brightness might not be better than one based and hue and saturation. Perhaps one should make the color space border out of hues with saturation 50% and then vary brightness from the border to the center (which would then be black) instead of varying saturation toward the center. All the different saturation values would than be projected into one point instead of projecting all brightness values into one point. Perhaps this would cover more colors as we subjectively know them. Colors such as brown, which is dark orange. On the other hand distinction between for example red and pink would disappear.

Camouflage

"Colorblind" people were used in World War II spy planes to spot camouflaged German camps that could usually not be spotted by people with normal color vision. Some people assume that the reason for this is that "colorblind" individuals are supposedly more trained in looking for outlines instead of colors than other people. I don't think this is true. It's only that there are circumstances where the outlines are easier to detect for "colorblind" individuals (in other cases it's easier for normal people). An example of this is the hidden-digit color vision dot test. The digit is made from randomly mixed dots of three different colors. The background is made from another set of randomly intermingled dots of three other colors. This effectively camouflages the digit to most people with normal color vision, who will see a seemingly random field of dots of lots of different colors. But the colors in the first set are chosen so that they look alike to a color deficient person. And the same is done for the second set. So the color deficient individual clearly sees the digit because the dots of the digit seem of similar hue which contrast well with the dots of the background which seem to have another single hue. The fact that the digit is invisible to most people with normal color vision is not a vision defect in a very strict sense, but due to a lack of complex processing abilities. We are well trained to distinguish two objects of different hues, but not to distinguish two random combinations of 3 hues, which is much more confusing. Given enough time to analyze the image an intelligent person with normal color vision will be able to decode the pattern and detect the digit. A few of them will even detect the digit very quickly, so this is not a reliable color vision test.

This may be the explanation why "colorblind" individuals were better able to spot camouflaged camps. Another possibility is that "colorblind" individuals were able to spot those camouflaged camps precisely because of their color discrimination abilities, not because of a lack of them. This may sound absurd at first, for how can someone with less color discrimination ability discriminate colors better? Well, this is entirely possible. Let me give an example. As I mentioned, I have a green weakness. The most reliable measurement of types of red-green deficiency is the anomaloscope, invented by Rayleigh in 1881. The subject is shown a color of pure yellow wavelength. Then he is asked to generate the same color yellow by adjusting two wheels which add green and red light to combine into yellow. I haven't done this test, but because of my green weakness I would add more green light than a normal color vision individual. Two conclusions can be drawn from this: (1) a normal color vision individual sees a difference between my generated yellow and the comparison yellow, for I have added too much green, and (2) if a normal color vision individual adjusts the green and red to generate a yellow indistinguishable from the baseline yellow I will see a difference between the two yellows, because compared to my settings his generated yellow contains too little green. In other words, my generated yellow looks a bit greenish to a person with normal color vision, while his generated yellow looks a bit orange to me.

This means that color visions tests could be made the other way around, without resorting to the camouflage trick described above. Most color dot tests are made so that people with normal color vision see numbers in them which cannot be seen by "colorblind" people. The tests could also be based on hues which can only be distinguished by "colorblind" people. An ability to see the numbers would then be proof of "colorblindness", while not being able to see them would be proof of normal color vision. As said before, the term colorblindness is relative in the sense that creatures with tetrachromatic color vision may call normal humans colorblind just as normal trichromatic humans call dichromatic and anamolous trichromatic humans colorblind. But reality is even stronger than that. There is a form a symmetry between humans with defective color vision and humans with normal color vision. People with normal color vision can label people with defective color vision as colorblind because the latter cannot distinguish certain colors which the former can. And people with defective color vision can call people with normal color vision colorblind because the latter cannot see certain color differences which the former can. On the whole, though, it must be admitted that people with normal color vision have a significantly greater color discrimination ability than people with color deficiency. By the way, a slight advantage of "colorblindness" is that "colorblind" individuals have better night vision on average than people with normal color vision.

The fact that "colorblind" people can see color differences which are invisible to other people, might have some more uses than being able to bomb the camps of Nazis who forgot to camouflage their camps for color deficient people as well as for people with normal color vision. As suggested by the example above, it would be possible to create color coded messages which are visible only to "colorblind" individuals. This is a variation of the invisible color coding mentioned before, but now without the need for decoding lenses. For example, one might paint with invisible (dark) yellow letters on a (dark) yellow background, which would be visible as orange on yellow by a green weak individual or green-yellow on yellow by a red weak individual. The effect might be especially large for red-weak individuals if the letters are made with green and a deep (long wavelength) red. In particular, dichromatic protans can't see deep red so for them there would seem to be a good contrast between darker letters on a lighter background. Perhaps a color deficient individual could use this ability to see things which others can't see so that he can pretend to be a psychic and claim the famous million dollar prize from James Randi.

Another example of a color vision difference one could make use of is the difference in violet perception. As mentioned before, normal color vision individuals can hardly tell the difference between spectral violet and bluish-purple. But I assume that dichromatic protons, who lack red cones, can tell the difference, since they lack cone sensitivity inversion in the violet part of the spectrum. So spectral violet should seem to them a sort of deep blue that normal people cannot see, while bluish-purple should look like regular blue (or greenish-blue) to them.

Subjective color perception

One might wonder whether, for example, dichromatic deutans, who lack green cones, might perceive a more intense, deeper, more saturated kind of red than normal people. Their red is not contaminated with green cone signal as it is with normal people, who in some sense always see a red contaminated by green and a green contaminated by red. I suspect this is not correct, however. Note that people with normal color vision get all the same blue/red cone signals as dichromatic deutans. They only differ in that they get additional information not available to dichromatic deutans: green cone information. It does not seem reasonable to assume that adding information while taking no information away would make a color seem less intense. One could wonder though if one would be able to see new colors if a pill were invented to temporarily anaesthetize two of the three cone signals, creating a single pure cone signal which normally never occurs due to sensitivity curve overlap.

Below you will find some links creating a simulation of how "colorblind" people see things. These things are good for an impression of what the color space of a color deficient person looks like and to simulate color contrast. But I would be less certain that they give a good idea of the subjective color perception of the color deficient. The fact that my green and red are a bit mixed up doesn't necessarily mean I subjectively see my reds and greens as brownish. From the fact that I see less difference between red, green and brown than a normal color vision individual it cannot be determined whether I see my reds and greens as brownish or rather my browns and greens as reddish or my browns and reds as greenish. Perhaps it is more correct to say that I do see pure red, green and brown, only they look more alike just as a normal person might see them with less contrast in a badly lit room without necessarily turning them into other colors. Color vision is defined by a difference between colors but it is hard to define colors absolutely and I doubt whether any such questions are even valid in principle. In any case I can assure you that the colors of the world look exactly right to me.

It is often said that color deficient people people tend to confuse colors. Sometimes the term CVC (Color Vision Confusion) is suggested as an alternative for the term colorblindness. I don't think that is a good term and I suppose the terms "color deficient", "color vision deficiency", "color weak" or "a little bit colorblind" would be better. Although it is true that color deficient people tend to confuse colors clearly distinguishable by normal color vision individuals, color confusion is not their fundamental distinguishing characteristic. What distinguishes color deficient individuals from normal color vision individuals is a smaller color space. But within their own color space normal color vision individuals are equally prone to color confusion as color deficient individuals. Just as a red-green color deficient individual might confuse brown/red, brown/green, orange/red, green/grey, etc., so too a person with normal color vision might confuse red-orange/red, bluish-green/blue-green or bluish-purple/purple. It's only the color vocabulary that suggests that normal color vision people are less prone to confusion within their color space. If bluish-green were called green and if blue-green were called blue, suddenly a confusion between bluish-green and blue-green would sound much worse, since the names suggest two entirely different colors. Conversely, if brown were called reddish-brown and if red were called red-brown, then a confusion between brown and red wouldn't sound too bad, because their names would be so alike. So color confusion by color deficient individuals is caused by a vocabulary based on normal color vision individuals. If they created their own vocabulary they would be just as unlikely to make errors as normal color vision individuals, at least within their own color space. Of course if color space is defined according to the complete set of possible light spectra, then normal color vision individuals as well as color deficient individuals suffer from massive color confusion. Interestingly, using the vocabulary of normal color vision people probably affects the color perception of color deficient people. For example, I tend to see more difference between orange and red than between purple and reddish-purple, even though the difference is probably about the same in my color space. Presumably this is because the greater naming difference suggests a larger color difference and therefore I have been trained more to make that distinction. This is at least true for absolute color judgements (naming a single color) and probably not for relative color judgements (when I can see two colors next to each other).

Is "colorblindness" a handicap? Well, it is a very slight artificial handicap of sorts, created by the color codings invented by people with normal color vision. The only true handicap I have related to by my "colorblindness" is an inability to tell if someone is blushing. I don't have much of a problem eating unripe fruits (evolutionary theory has it humans developed color vision to tell when fruit is ripe, to tell which plants are poisonous and to help defeat animal camouflage). All other problems I might have with my color defect are caused by the fact that human color coding is often based on the majority of people with normal color vision. So I have a bit more trouble with color coded maps and color coded electronic components. I don't mind those sort of things much and they hardly create a problem for me. There is no good reason why other people should limit the richness of their color coding schemes just for the sake of the minority. The only thing which I think is too bad is that there are still some places using red traffic lights of a pure deep red, which are almost invisible (black) to a protan dichromatic, which is like creating an infrared traffic light for a normal person. This is dangerous, especially in rare cases with single light traffic lights such as for trains. I also question the rationale of excluding 8% of males from certain professions where very little is needed to make the job suitable for them. I would think, for example, that the coding schemes used for pilots and ship captains might be simple enough that they could be made without using red-green coding. The only real disadvantage I ascribe to my color deficiency is in terms of esthetic value. I believe a world with richer colors would be slightly more esthetically pleasing.

A common example given of a minor problem for color deficient persons is that they might incorrectly pair their socks after washing them. The main theme of this article can be summed up as the note that the normal person suffers from exactly this same problem, except nobody notices this (except perhaps the colorblind).

Links

Color vision perception
Color vision - detailed explanation
The CIE color model
Out of Gamut: Why Is Color, CIE color model.
The hue/saturation/brightness color model
CIE model, color principles - hue, saturation, and value
Additive and subtractive color mixing
What the world looks like to the "colorblind"
What strawberries look like to the "colorblind"
What apples look like to the "colorblind"
Easy color vision test
Ishihara test for "colorblindness"
What is Color Blindness?
Colorblindness
Simulate how a "colorblind" person sees colors
Colorblindness theory, simulation
Color illusions
More color illusions
We are ALL colorblind
How colorblind people see the world
Vischeck simulates colorblind vision. Daltonize corrects images for colorblind viewers.
Confusion Lines of the CIE 1931 Color Space
What type of color blindness do I have?
RGB Anomaloscope — How severe is my Color Blindness?

Email: henry@sturman.net