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

Question

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The opponent-process theory suggests that we perceive color in terms of paired opposites: red versus green, yellow versus blue, and white versus black. People with red-green color deficiency have a reduced ability to distinguish between red and green, but the opponent-process theory explains that they can still distinguish yellow, which is a combination of red and green opponent responses.

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