Rainbows and spectra

Red + Green = Yellow

Green + Blue = Cyan

Blue + Red = Magenta

These additive secondary colours are in fact names of colours used as subtractive primaries.

Additive primary colours

When coloured lights are added together the mixture is always lighter. Additive primaries red, green and blue when combined in the correct intensities produce white light.

Red + Green + Blue = White

If we add two of the primary colours together a secondary colour is produced. For example, if we mix red and green light we obtain the secondary colour yellow.

Almost all colours can be made by mixing different amounts (intensities) of the additive primaries together.

Some of the colours that can be produced do not appear in the rainbow, for example pink, magenta and purple.

We do not actually see the coloured light itself, only its reflection from a white surface.