Cones and opponency.
D000818:Animals; D003118:Color Perception / Q000502:physiology*; D005829:Geniculate Bodies / Q000502:physiology; D000882:Haplorhini; D006801:Humans; D010786:Photoreceptor Cells / Q000502:physiology*; D011601:Psychophysics; D014793:Visual Cortex / Q000502:physiology