The ancestors of modern humans developed color vision 30 million years ago. But it was not until the late 1700s that there are records of anyone seeing colors in an unusual way. English chemist John Dalton, who found that people thought he was joking when he asked whether a geranium flower was blue or pink, wrote a description in 1794 of what he saw for the Manchester Literary and Philosophical Society journal: His world was suffused by shades of blue and yellow, but contained none of the mysterious sensation known as red. “That part of the image which others call red,” he wrote, “appears to me little more than a shade or defect of light.” It was one of the first mentions of colorblindness in human history.
In the centuries since, we have discovered what it is that robs some people of such sensations. Those of us with standard vision, called trichromats, have three kinds of pigments, or cones, in our retinae, each sensitive to a certain range of light and spaced out across the visible spectrum so that they can together convey to the brain everything from red to violet. In the colorblind, a mutated cone is so close to another in sensitivity that parts of the spectrum aren’t covered, or there are only two functioning cones, a condition called dichromacy. A difference of one cone causes a serious change in the number of discernable colors: Dichromats see on the order of 10,000 colors, trichromats on the order of a million. But that isn’t the end of the story. Recently, as genetic analyses and tests of color vision have grown more sophisticated, we are stumbling into one of the most curious discoveries in vision since Dalton’s day. Dichromats have 2 cones, trichromats have 3, tetrachromats have 4, making them theoretically capable of seeing 100 million colors.