Most of us wonder, at one time or another, how the world appears in other people’s eyes. Does the red I see look like the red you see? If not, how are they different? If you are not colorblind, you have probably wondered what the world looks like to those who are. You have probably seen pictures that approximate what it looks like, but you will never know. If you are colorblind (or color deficient, which is a more accurate term), you have probably wondered what you are missing. I can’t answer this for you in a satisfying way but I can tell you that technically the answer is not much at all.
If you compare the sensitivity of the human eye to that of a mantis shrimp, we all turn up highly color deficient. The world is full of colors none of us will see, whether we have a standard set of human rods and cones or not. I should accept this fact and move on, but I can’t. I want to know what a color wheel would look like if a mantis shrimp needed to use one.
Do you remember the color wheel? I see it practically every day, using PhotoShop. It usually looks like this:
It probably looks normal to you. It looks very normal to me. In fact, my brain works every day under the assumption that these are all the primary colors and all 
the basic ways to blend them. (For simplicity’s sake, I’m going to ignore browns and less saturated colors for the moment.) My brain is also familiar with the reality that there are wavelengths beyond what I can see, like ultraviolet light, x-rays and infrared. My brain even understands that these unseen colors live above and below the rainbow, when rainbows are around. If you haven’t considered this before, take an extra minute to think about it next time you see a rainbow.
Where I get tripped up is envisioning the standard color wheel as it might appear if I could see even one more primary color — say, the near ultraviolet spectrum. (This would make a me a functional tetrachromat, and, at present, only one such person has yet been found and it isn’t me.)
On a rainbow I know the shades of near ultraviolet light live right under the purples I can see. But where the heck does the extra color go on a color wheel? The answer is more complicated than it appears.
On a color wheel, the spectrum of visible light is wrapped around from violet to red, so that when violet and red meet (from the top and the bottom of the rainbow), they blend and make some nice magenta and other slightly violet reds.
But, see, we know that the top and the bottom of the rainbow are only the visible top and the visible bottom. That topmost red and that bottommost violet are only defined by our eyes. It I could see those colors in the the near ultraviolet spectrum, they would wrap with the topmost red. If I could see into infrared, I would have to bump squeeze in even more. Maybe this doesn’t seem like a big deal, but stick with me:
If I stick a placeholder color in for the near ultraviolet spectrum, a black to represent a color I can’t see and PhotoShop can’t reproduce, it begins to eat away at the nice magenta as it blends because that red and violet don’t get to meet anymore. Something has come between them.
We know my new placeholder color represents a placeholder color that lives under violet. So what color does it represent, on the top most part, where it is blended in with that red?
Think about this for a second and it grows more confounding. We can mix colors together. All of us have experience with this as children. Even if many people give up on color mixing in adulthood, they still understand the principle that yellow and blue, for example, yield green. So what does near ultraviolet mixed with green look like? Or near ultraviolet mixed with red, or mixed with blue, or even mixed with plain old regular purple?
The answer, of course, is we don’t have any clue. That’s fine, I guess, but even so, how does a color wheel work with this extra color? Does it need an extra dimension instead of me wedging it in? Is the color wheel actually based on some mathematical principle for only three primary colors? If you add a fourth, does it require a rhombus, a blob or sphere, or something?
And what the heck happens if you are an artistic mantis shrimp, who can see a staggering 11 primary colors? How do you form that color wheel? Does it have to be extra dimensional?
But even more taxing for my poor mind is the knowledge that the three primary colors that I know so well are essentially arbitrary. Humans evolved three photo receptors that respond to three ranges of light.
I don’t know what the range of variation is from human to human, but I do know our receptors are basically the same. Even the people who are color deficient and lack one or more cones, and that one woman who is a tetrachromat, they still are seeing in the same range sensitive to approximately the same range of light. The big differences, if they exist, likely have more to do with how the brain interprets the information it is getting than with the sensitively of the eye.
So the blue sky we live under (when the grey clouds pass) and the green grass we walk on (when we city folk stray from the pavement) are colors experienced specifically by humans, unique to our human eyes. I may feel adrift, thinking of all the colors I’ll never see, because we are all woefully color deficient compared to the mantis shrimp and even that noble, color-gobbling crustacean is still seeing a fraction of available light. But the colors we get to see are like a fingerprint of our shared experience.
To put it tritely, we all bleed red, and how we see that red is uniquely human.