Why do humans see most colours through the combined activity of different cone cells?

Humans perceive most colours through the combined activity of different cone cells because the human eye functions as a trichromatic system. The retina contains three distinct types of cone cells, often designated as S (short), M (medium), and L (long) based on their peak sensitivities to blue, green, and red wavelengths of light, respectively.1234

Rather than responding to a single specific wavelength, a given colour stimulus typically excites all three types of cones with varying degrees of effectiveness. The brain interprets the resulting pattern of relative excitation—the ratio of response across the three cone types—to determine the specific hue perceived. For example, a yellow light stimulates both the red (L) and green (M) receptors while having little effect on the blue (S) receptors; the brain combines these signals to produce the sensation of yellow.324

This mechanism allows the eye to discriminate among hundreds of different wavelengths and subtle changes in the environment. By relying on the relative intensity of responses rather than individual receptors, the visual system effectively reduces the possibility of confusion between stimuli of different intensities and wavelength compositions.32