I am not colorblind, but because of my profession, people often ask what it means to be colorblind since that is one of the common visual deficit manifestations that they can relate to. Color blindness (pretty good article on Wikipedia there) is very common and just about everyone knows someone who does not see the world quite like they do because of a form of color blindness. The animated gif above shows the red, green and blue pixels on an LCD display as they are seen by a trichromat and a person with two of the common forms of color blindness, protanopia and deuteranopia. In essence, there is red and green confusion with red and green both appearing as different shades of yellow.
Here is a more natural scene, courtesy of David Hobby with people with differing skin tones and different color clothing in a lush, green background, the Viñales region of Cuba from this entry. People with trichromatic vision see the image in full red, green and blue glory. We see the blue of the sky, the green foliage against the orange volcanic soil and the varying hues of shirts in this image.
However, this is how a person with color blindness sees the world. In protanopia, there is a complete absence of red cone photoreceptors. The sky is still blue, vegetation is olive green and everyone skin looks deeply olive colored as well. The difference with deuteranopia is subtlety different. In deuteranopes, the green cone photoreceptors are absent, moderately affecting red-green color discrimination. Pay attention to Laura’s red shirt in the trichromatic image and note that the protanope or deuteranope will have a tough time discriminating that hue from the green of the foliage. Or look at the orange in the clothes of Waldemar on the far left and compare that to the green foliage. You can see how someone would have a tough time discriminating those hues. Furthermore, the concept of “purple” is a tough one to communicate. If you look at David’s purple shirt in the trichromatic image, it appears as a shade of blue to a protanope or deuteranope.
There is another form of color blindness called tritanopia where the blue cones are missing. The effect appears like some of the old color slides that are starting to fade by losing the blue pigment. This does not mean they cannot see blue as they get the percept from the color mixing of the other channels, but tritanopes, like protanopes and deuteranopes are dichromats. Many colors are easier for a tritanope to figure out, but the purple in David’s shirt looks like dark green and the light blue in my shirt appears light green to a tritanope. This is important as most folks who are color blind can actually see color. Though there are forms of color blindness that are extremely rare where there is complete color blindness, a condition called achromatopsia.
So, you can see how color confusion can be a problem and why when displaying color graphics, its such an issue for some folks. In those with profound color blindness for instance, they may only be able to discriminate 20 separate hues. So, if you are designing graphics, its not a bad idea to test them to ensure that they can be seen by folks with color blindness, given that upwards of 8% of all men have a form of color blindness. There are a number of ways of proofing images, and given Adobe’s moves towards “rented” software through subscriptions, I am loathe to recommend them. However, Photoshop is a commonly used tool and I commonly use Photoshop to check graphics to ensure they can be seen and interpreted by individuals with the two most common forms of color blindness with soft proofs. You simply go to View > Proof Setup > to find two options for protanopia and deuteranopia. This is also something that is important for website design. There is also a very cool little app, available cross platform called ColorOracle that also includes options for viewing images as a person with the very rare tritanopia will perceive them.
Incidentally, since we are talking about color perception, there are two other interesting variations on the theme that many folks might be able to relate to: 1) Lots of folks think that dogs and cats see the world in black and white. They, being dichromats, actually see the world in color very much like what a “color blind” person can see. They do not see only black, white and variations of that in grey, but do not have the same resolution as we do. Nickolay Lamm did a spectacular job of representing how cats see the world from the perspective of color, optics and resolution here in his post, What Do Cats See?
Additionally, one might ask if you could go in the other direction, towards more color discrimination and it turns out you can. Jay and Maureen Neitz have done pioneering work in the mechanisms of color vision and it turns out that there is some evidence that some females see the world as tetrachromats… So that joke of the man asking if the tie matches and the woman insisting it does not, has some biological merit. In effect, because the genes for color are carried on the X-chromosome, a gene duplication event or a shift in the encoding for an opsin may in fact result in two copies of say… the green cone, both at slightly different wavelengths. The result would be that a woman sees in blue, red and *two* wavelengths of green.
Before I leave you though, consider that human color vision is not the end all, be all. We tend to be egocentric about things relating to evolution, but the reality is, most other vertebrate species on the planet see color better than we do. Many fish, birds and insects are nominally tetra-chromatic with many species having additional spectral filters at the ends of photoreceptors in the form of oil droplets. It is likely that many of these species are pentachromats and some amphibian or reptile species with longer discontinuous evolution of their color visual systems may be able to discriminate even more hues. The all time winner in the known animal kingdom is the mantis shrimp that appears to perceive a truly multispectral world as dodecachromats. That’s right… nominally, 12 kinds of photoreceptors in their eyes. Push that around in your head a little… Vision is a powerful advantage in evolution and the Oatmeal has a wonderful cartoon illustrating the competitive advantage vision gives the Mantis Shrimp.
Hat tip to Tim Wesson for the Oatmeal cartoon.