How is mixing the colors of paint different from mixed colors of light?

Answer 1

Sunlight is a great thing.

When you use a prism, you can see how it splits up the sunlight into all the colours of the rainbow. This is because the prism splits up the different wavelengths of the light.

All wavelengths combined makes us see the light as white on a piece of white paper. This demonstrates the additive property of light.

Paint however behaves differently. Paint does not really have a colour at all. Nothing in the world does really have a colour except for the light. We only see the colour of the light that it is being reflected from surfaces.

We use different pigments in paint in order for it to absorb all the unwanted wavelengths and to reflect only the desired wavelengths of light. Pigments therefore work by subtracting some of the light and only reflecting the remainder.

So the difference between mixing light and mixing pigments is that the first is additive and the second is subtractive.

The primary colors of light are red, blue and green, whereas the primary colors of pigment are cyan, magenta and yellow (however, many painters still mix with red, blue and yellow paints). So when the three primary colours of light (in equal intensity) are made to converge together, the mix will be seen as white light - an additive effect.

Since the true primary pigments (subtractive) are cyan, magenta and yellow, colour printers use these colours.

Red-blue-yellow is an obsolete triad that cannot produce the full range of secondaries. Painters have historically used R-B-Y to produce some of te colours they need, but commonly use extra colours to produce tints that their R-B-Y selection cannot create.

See Wikipedia for a complete explanation.

A perfectly subtractive system should produce black with an equal combination of all three primary pigments. This is difficult to achieve, usually creating a very dark muddy reddish-brown (though not what we commonly conceive of as brown). This is why colour printing commonly uses the "four-colour" process, adding black where true blacks or very accurate greys are needed.