Two people with normal sight can have strikingly different colour sensors
Your sense of colour comes from three kinds of cone cell, tuned to long, medium and short wavelengths. Yet the mix varies wildly: one man's retina was measured at about 76 percent long-wave cones, another's at roughly 51 percent, and both saw colour normally. The brain builds a consistent world from uneven parts.
Cones are the retina's daylight receptors. A human eye holds about six to seven million of them against roughly 92 million rods, which handle dim light. Cones are less sensitive than rods but react faster and resolve finer detail, partly because each one has its own dedicated line to the optic nerve. They crowd most densely into the fovea, a patch just 0.3 millimetres wide with no rods at all, which gives us sharp central vision.
Humans normally carry three types. L cones respond best to longer, reddish light, peaking around 560 nanometres; M cones peak at 530, in the yellow-green; S cones peak at 420, in the blue, and make up only about 2 percent of the total. The brain compares their signals: a little more L than M reads as yellow, a lot more reads as red. Because M and L respond almost equally to yellowish-green, that is the colour the eye picks up most easily in bright light. Rods, by contrast, peak at 498 nanometres, about midway between S and M.
The pigments themselves vary with genetic mutation, so individuals differ in exactly how their cones respond. Losing or shifting one cone class produces the familiar red-green colour blindness, while having none that work leaves only rod vision. At the other extreme there are some verified reports of people with a fourth cone type and tetrachromatic vision.
Cones also tire. Stare at one colour for about a minute and the cones tuned to it become exhausted, so looking away produces a vivid afterimage in the opposing hue that can linger for a minute or more. Each cone has a light-catching outer segment shaped like a cone, which gives the cell its name, stacked with membrane discs that wear away and are recycled.
Source: Cone cell
