Today many people know, that there is an RGB colorspace. Some even know, that one usually uses a CMYK colorspace for printing.
But how do we actually see colors? Here comes a very short explanation:
The human eye has in it’s retina cells, which are lightsensitive. These cells do not perceive/interprete light in the same way, and that makes it possible for us to see colors. There are a couple of theories about how, and these are just above a 1oo years old. Today they are both seen as the explanations for different stages of clor perception.
In the eye there are two kinds of cells: Cones and rods. The cones make us see colors.
According to one theory the cones come in three kinds, each receptive to a part of the light spectrum: long wave (l) or reddish colors, medium wave (m) or greenish colors and short wave (s) or bluish colors.
As can be seen in the illustration, which shows curves of sensitivity within the spectrum, Red and Green are very close and overlapping. Red is actually topping in the yellow area.
The other theory is more about how the eye and the brain interprete the incoming impulses.
This theory says the eye balances colors against each other. Red against green, yellow (which is a combination of red and green) against blue and lighthess against darkness,
According to this, what we perceive are the 6 primary colors: Red, Green, Yellow Blue White Black(dark).
May be one should say reddish, greenish and so on. It is beleived, that each type of cone perceives or is able to separate about 100 shades. If the brain is able to combine these shades exponential that a human eye can see/separate about one million shades of color.
Like everything else in nature, there are indiviual variations in the cells and a persons brain, when it comes to receiving and interpreting these colors.
Also see Color perception.