The center dot at this exhibit is pure yellow light. It’s surrounded by a ring of 11 dots of varying colors, created by mixing different proportions of red and green light. (Surprise: Red and green light mixed together appear yellow to our eyes.)
Exactly which of the red/green dots seems to best match the yellow dot varies from person to person. Thanks to variations in the pigments of our retinas, people perceive color differently. One person will see a match between colors that another person thinks are very different.
Variations between people in their perception of color reach an extreme in color blindness. About 1 in every 12 men (and 1 in every 200 women) are colorblind, the common name for a condition more correctly called color deficiency.
Unlike ordinary human trichomats, who have three types of color-sensitive cones, most color-deficient people are dichromats, having only two types of color-sensitive cones, one most sensitive to blue light, the other to yellow light. The upshot is that colorblind people see the world in shades of blue and yellow—with red and green being especially hard to tell apart.