Short answers, pulled from the story.
Color is the visual perception produced when light activates the different types of cone cells in the eye. Color is not an inherent property of matter; it relates to how an object absorbs, emits, reflects, and transmits light.
It has been estimated that humans can distinguish roughly 10 million different colors. The eye reduces all incoming light to three color components using three types of cone cells, a condition called trichromacy.
Humans have S cones most responsive to blue or blue-violet light around 450 nanometers, M cones most sensitive to green around 540 nanometers, and L cones most sensitive to greenish yellow around 570 nanometers. Each cone's output is governed by the principle of univariance.
The most common form is congenital red-green color blindness, which affects roughly 8 percent of males. In its strongest form, dichromacy, blue and purple, green and yellow, teal, and gray appear as colors of confusion.
As many as half of all human females may express four cone classes in the retina, but only one peer-reviewed report of a functional tetrachromat exists. While the average person sees about one million colors, a functional tetrachromat could see a hundred million.
Additive color mixes light using red, green, and blue primaries in devices like projectors, televisions, and computer terminals. Subtractive color uses dyes, inks, pigments, or filters to absorb certain wavelengths, leaving the unabsorbed parts of the spectrum visible.
Thomas Young proposed the trichromatic theory in 1801, suggesting color derives from the mixing of three receptor outputs. James Clerk Maxwell confirmed it experimentally, and Hermann von Helmholtz later refined it.
How the continuous spectrum is divided into named colors is a matter of culture and historical contingency. In their 1969 study, Brent Berlin and Paul Kay found that languages with two basic color names split dark and cool from bright and warm, with red, yellow, and green typically emerging next.