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How do cone cells enable humans to see different colours?

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details of the mammalian eye — Photoreception The mammalian eye has a cornea and a lens and functions as a dioptric system, in which light rays are refracted to focus on the retina.

Humans perceive different colours through the function of three distinct types of cone cells in the retina, a process described by the trichromatic theory.

Each cone type contains a specific opsin protein that makes it maximally sensitive to a particular region of the light spectrum:

S (Short-wavelength) cones: Sensitive to blue light (peak near 419–445 nm).12
M (Medium-wavelength) cones: Sensitive to green light (peak near 531–535 nm).12
L (Long-wavelength) cones: Sensitive to yellow-green or red light (peak near 558–565 nm).12

Colour vision arises from the brain interpreting the relative ratio of excitation across these three cone types. For example, equal stimulation of all three cones is perceived as white.21

Beyond this initial reception, the retina further processes these signals through "opponent" mechanisms, where neural cells combine inputs to emphasize colour differences and contrast, ensuring that the information sent to the brain via the optic nerve is effectively colour-coded.3