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 color vision is based on three pairs of opposing receptors: red-green, blue-yellow, and black-white. Individuals with color-deficient vision for red and green have a malfunction in the red-green receptors, but their blue-yellow and black-white receptors are still functioning normally. Therefore, they are still able to perceive yellow through the blue-yellow receptor pair.

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