Red, green and blue - the three primary additive colours.
There are three types of cones in the retina of most humans' eyes. Each type has a peak response frequency, where light of that particular frequency causes more stimulation than light of any other frequency with the same intensity. One type of cone has a peak response in the red, one type has a peak in green, and the third peaks in blue. In this way, additively mixing light sources of these three colours should theoretically give rise to the whole palette of colours detectable by most human eyes. Equal intensities of red, green, and blue give the illusion of a pure grey or pure white colour.
The opposite of RGB is CMY - cyan, magenta and yellow. Cyan is the complement of red, magenta is the complement of green and yellow is the complement of blue. A complementary pair of colours are two which, if mixed in equal amounts, appear pure black, pure white or a pure grey. Another way of thinking of it is - cyan is green and blue, magenta is red and blue, yellow is red and green.
There are some women who are tetrachromatic&hellip they can see four distinct colours. As well as two types of cone which peak in red and blue, there are two more types which both peak in green, but at slightly different frequencies. Thus, tetrachromats do not get as much colour information from a "colour" CRT or LCD as they can by simply looking at the world, and essentially have four primary colours while most of the rest of us have three. Tetrachromats are more likely to have colour blind (bichromatic) sons than trichromats (and presumably they are more likely to have tetrachromatic daughters, though I have seen no evidence of this yet).