Seeing hue means comparing light frequencies
Color vision is the knack of telling lights apart by frequency mix, not merely by brightness. Photoreceptor types fire differently, then layered neurons build a hue. Newton showed a prism can split white light and a second prism can knit it back; human vision spans roughly 380–740 nanometers.
Outside that window lie infrared and ultraviolet—invisible to typical human eyes yet available to other species. Within the visible band, just-noticeable wavelength differences shrink to about one nanometre in blue-green and yellow regions and stretch to ten nanometres or more toward deep red and short blue. Even so, people can discriminate up to a few hundred hues under good conditions.
The pipeline is not a camera filter but a comparison engine: differential stimulation of receptor classes, then cortical construction of colour as a percept. Intensity and wavelength interact, which is why the same physical spectrum can look different under shifted illumination if the visual system fails to discount the illuminant.
Which receptors do the work depends on light level. In dim conditions rod cells take over; they peak near 500 nanometres and contribute little or nothing to colour, their rhodopsin supplying shades of grey. In daylight, cone cells carry colour vision, and humans have three kinds, which makes us trichromats. They are labelled short, medium and long by where their sensitivity peaks, but the labels mislead: the so-called red cone actually peaks in greenish-yellow light, and the RGB model is a convenience rather than a literal map. White can arise from the whole spectrum or from just two complementary colours such as blue and yellow. Many colours are not spectral at all: pink mixes red with white, brown comes from orange with grey or black, and magenta-purples bridge the two ends of a spectrum that runs from violet to red in a straight line. Impossible colours are cone combinations nature cannot produce; if medium cones could fire alone, we would see a hyper-green. The earliest vertebrate ancestor carried four cone photopsins, and peak responses differ even among people with typical vision.
Source: Color vision
