ADDITIVE COLOUR MIXING
Red, Green, and Blue lights illuminate a screen. Each source emits pure red, green, or blue light (i.e. they are the additive primary colours). As the light beams converge and the colours overlap, they illustrate additive mixing, yielding the additive secondary colours (yellow, magenta, and cyan). At the area where all of the primaries overlap, the white light appears:
red + green light gives yellow light
red + blue light gives magenta light
green + blue light gives cyan light
red + green + blue light gives white light
We are fortunate that we can mimic all colours using the additive primaries (pure red, green, and blue). By varying the intensity of each light source we can get any shade of any colour. This somewhat similar to how TV and computer displays work, with each pixel having three colour components, with the mixing occurring in our visual systems.