We began this discussion of color vision by talking about color defects. As you think about the most common color defect, a red-green deficiency, is there anything about it that might present a problem for the Young-Helmholtz theory? Hint: It has to do with the perception of yellow. If the perception of yellow comes from stimulation of “red” and “green” receptors, and a person has a red-green color defect indicating a problem with “red” and/or “green” receptors, then that person also should have trouble perceiving yellow. However—and this is a major problem for trichromatic theory—people with a red-green defect have no difficulty with yellows and blues.

Critics of the Young-Helmholtz theory proposed a variety of competing color vision theories, the most successful of which was suggested by Helmholtz-contemporary Ewald Hering (1834-1918). Hering developed opponent-process theory, theorizing that the eye contains three receptor complexes that each respond in one of two opposing ways. For example, a red-green complex might be excited by red and inhibited by green or vice versa, whereas a blue-yellow complex might be excited by blue and inhibited by yellow or vice versa. Hering's third opponent process is a black-white complex that accounts for brightness perception (i.e., is excited by brightness and inhibited by darkness or vice versa).

Opponent-process theory better accounts for color defects than trichromatic theory. For example, because the blue-yellow complex is presumably intact, a person with a red-green defect should have no difficulty perceiving yellow, which, as we've indicated, is the case. Also, the color defects come in pairs, which we would expect if opponent-process theory is true. Although most people with color defects have difficulty distinguishing reds and greens, there are some people who have difficulty distinguishing between blues and yellows.