People with color-deficient vision for red and green may still see yellow.This is most easily explained by

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The opponent-process theory suggests that our color perception is based on three pairs of opposing colors: red-green, blue-yellow, and black-white. When one color in a pair is stimulated, the other is inhibited. People with color-deficient vision for red and green may still see yellow because yellow is a combination of red and green wavelengths that stimulate the blue-yellow opponent system without involving the deficient red-green opponent system.

People with Color-Deficient Vision for Red and Green May Still