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, so they illustrate additive mixing, yielding the additive secondary colours (yellow, magenta, and cyan). At the area where all of the primaries overlap, 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