What is additive colour matching?

Additive colour mixing refers to the mixing of different coloured lights and can be easily demonstrated by the superposition of lights (primaries) on a white projection screen. When this is done using red, green, and blue primaries, the colours yellow, cyan, and magenta are produced where two of the primaries overlap. Where all three primaries overlap the sensation of white is produced if the spectral distributions and intensities of the three primaries are carefully chosen.