ADDITIVE AND SUBTRACTIVE 
FORMATION OF
Colour

Variations of coloured light can be produced by adding or subtracting different wave lengths of light from the source. A roughly uniform mixture of all the visual wave lengths produce white light!

By taking just three areas of the spectrum

and mixing lights of these three colours in different proportions, any colour in the spectrum can be reproduced.

beam of light combined with creates
beam of light combined with creates
beam of light combined with creates
combined creates

This is called additive Colour

I used this additive principle for the multiple projection  enlighten  work which involved 7 infrared activated projectors each with a different colour filter.

This is an important principle, because it means colour film needs only to have layers sensitive to these three primary bands to be able to record images of all colours. But just as all colours can be formed by adding primary coloured lights, so you can start with white light and subtract various quantities of primary colour.

For example, to subtract only blue you would place a yellow filter in the light beam, because this allows both green and red light to pass. 
To subtract green you use a bluish - red (magenta) filter.

To subtract red you use a green - blue (cyan) filter.

So yellow, magenta, and cyan are anti or complementary colours to blue , green and red respectively. This inter - relationship is VITAL for the understanding of colour processing and printing.