OPPONENT PROCESS COLOR THEORY

Hering 1878 · Hurvich & Jameson 1957 · Retinal & cortical opponent channels

Opponent process afterimage:
Stare at the colored square for 5 seconds, then look at the gray test patch. You'll see the opponent color:

Red adapts → Green afterimage
Green adapts → Red afterimage
Blue adapts → Yellow afterimage
Yellow adapts → Blue afterimage

Mechanism: Sustained firing of R−G or B−Y opponent neurons causes fatigue. On neutral stimulus, the non-fatigued opponent channel dominates.
Three opponent channels:
L−M (red-green): Long − Medium cone response
S−(L+M) (blue-yellow): Short − luminance
L+M (luminance): Achromatic channel

Unique yellow (~580 nm): zero R−G signal.
Unique green (~540 nm): zero B−Y signal.
These channels are orthogonal — no simultaneous red-green or blue-yellow percept exists.
CIE 1931 chromaticity diagram with opponent axes overlaid.

The red-green axis and blue-yellow axis are opponent — moving along one axis doesn't change the other channel's signal.

Click on the CIE diagram to see the opponent coordinates of any color. The diagram is drawn in perceptual (CIECAM) approximation.
Unique (unitary) hues — the four colors that appear "pure," containing no trace of another hue.
Unique Red (~700 nm or ~450 nm): no yellow, no blue tinge.
Unique Green (~530–540 nm): no red, no blue.
Unique Yellow (~580 nm): no red, no green — exactly where L−M=0.
Unique Blue (~470 nm): no red, no green.

These four hues are linguistically universal (Berlin & Kay 1969): every language with 4+ color terms has words for them. Opponent theory predicts exactly these zero-crossings.