Opponent Process Theory

Your visual system does not process colors independently. It uses an opponent process: three channels that each measure the balance between two competing signals. One channel weighs red against green.

Each channel works like a tug-of-war. A signal can be red or green, but never both simultaneously. Blue or yellow, never both. Magenta activates the red channel and the blue channel with no green opposition — a combination that has no single-wavelength equivalent in nature.