Colour Primaries and Colour Vision

There are two systems of colour mixing and each has different primaries.

Subtractive colour mixing
If one asks a school child who has started painting which are the three primary colours, the answer will most likely be blue yellow and red. Later in life, if still interested in colour, the answer would be cyan, yellow and magenta. These colours, as used by the colour printer, give the largest possible gamut of surface colours. The colour mixing system to obtain these surface colours with pigments is known as subtractive. That is each of the component pigments is subtracting some part of white light. The mixing of transparent dyes to make coloured gelatine filters is also a subtractive process when each of the component dyes subtract some part of the transmitted light.

Additive colour mixing
The other system is known as additive and has different primaries. It is the addition of coloured lights and may be demonstrated by the use of three projectors, each with a different coloured filter and a white screen. It is found that the largest colour gamut on the screen is to be obtained by using red, green and blue lights. In this system, yellow which is well known to be an absolute primary in subtractive mixing is a secondary colour formed by adding red and green lights. Indeed, the primaries of one system are the secondaries of the other and in general the following mixing diagram holds true:

When mixed in pairs the primaries of one system which are then secondary colours in that system become the primaries of the other system. If all additive primaries are mixed together in the correct quantities, the result is white light. The subtractive primaries so mixed yield black.

For one who is used to subtractive mixing the concept of additive mixing is a little confusing as yellow and blue lights yield white, not green (yellow, of course, is a secondary composed of red and green. When blue is added the eye believes the spectrum to be complete and sees white.