Red, Green, Blue. The three colours of light which can be mixed to produce any other colour. Coloured images are often stored as a sequence of RGB triplets or as separate red, green and blue overlays though this is not the only possible representation (see CMYK and HSV). These colours correspond to the three "guns" in a colour cathode ray tube and to the colour receptors in the human eye.
Often used as a synonym for colour, as in "RGB monitor" as opposed to monochrome (black and white).