Color Vision

Trichromatic theory explains the first stage. The retina has three types of cone cells, each most sensitive to a different wavelength: short (bluish), medium (greenish), or long (reddish) light. Every color you see comes from some mix of signal strength across these three cone types, the way a TV screen builds millions of colors from just red, green, and blue pixels. Three cone types are enough because the brain reads the ratio of activation across them, not just whether light is present.

Trichromatic theory alone cannot explain afterimages, though. If cones just got tired from overuse, staring at red should make red look weaker or grayer, not make you see green. Something has to actively produce the opposite color. That something is opponent-process theory: past the retina, neurons are wired into three opposing pairs, red-green, blue-yellow, and black-white. Each cell in a pair gets excited by one color and inhibited by its opposite. Stare at red long enough and that channel's red response fatigues; when the red square disappears, the channel rebounds and fires the green signal instead, painting a green afterimage in the same shape on a blank wall.

The two theories describe two stages of the same system rather than competing explanations, which is why damage shows up differently depending on where it happens. If a cone type is missing, trichromatic mixing breaks down. If a specific opponent channel malfunctions, as in red-green colorblindness, the person loses the ability to tell red from green specifically, while blue-yellow and brightness perception stay intact. This sets up later topics on sensation versus perception, since color vision shows raw receptor input being actively reorganized by the nervous system before it ever reaches awareness.