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 the visual system processes colors in three opposing pairs: red versus green, blue versus yellow, and black versus white. Therefore, individuals with color-deficient vision for red and green may still perceive colors along the blue-yellow and black-white dimensions, such as yellow.

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