Primary colours have nothing to do with the nature of light. It depends on how we humans detect light. Colour refers to a sensation that a particular wavelength of light produces.
Human eyes have two types of cells for absorbing light: rods and cons. The rods used mostly for night vision, detect primarily light and dark. The cons detect colour and favour some colours over the other. They absorb blue, green and red lights best. Hence they are named primary colours.
Primary colors (or primary colours) are sets of colors that can be combined to make a useful range of colors. For additive combination of colors, as in overlapping projected lights or in CRT displays, the primary colors normally used are red, green, and blue. For subtractive combination of colors, as in mixing of pigments or dyes, such as in printing, the primaries normally used are cyan, magenta, and yellow, though the set of red, yellow, blue is popular among artists. The combination of any two primary colors creates a secondary color.
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