Additive colour

The additive colour theory applies to colour mixing with light sources (eg, lights, tv, computer screens, etc). The three primary additive colours are blue, red, and green. Secondary colours are yellow (obtained by mixing red and green), cyan (obtained by mixing green and blue), and magenta (obtained by mixing blue and red). Mixing all primary colours will produce white light.

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(Image embedded from Wikipedia on 2 January 2011)

The additive primary colour theory is behind modern electronic colour management processes, such as the RGB colour model.

Some artists also argue that colour painting techniques where pigments are not mixed (eg when using crayons, or the technique used by some impressionists and pointillists) rely on the eye "mixing" the colours in an additive manner.

One basic property of additive colour mixing is that any mixture will produce a brighter colour than the initial unmixed ones. Another property is that the luminosity and saturation of colours also depends on the background colour where the light is projected, the intensity of the light, etc.

This has important practical consequences nowadays, given the proliferation of electronic displays in our life. For example, an electronic photograph or an internet picture will always look much brighter than the same photograph or image printed either in photographic or normal paper: a recipe for disappointment, unless well understood and accepted. Also, the "quality" of the image will vary, for example, with the type of monitor used and the graphic card's capability to manage colours. Thus, the same "colour" may look different in two different monitors.

Desktop printers are optimized to work with RGB, and will convert them into the best CMY print possible. However, if you are intended to use an offset press, a way of "managing" this property in order to have a better idea of how colours will print in a subtractive environment is to convert RGB colours into CMYK colours on your computer screen. You are still seeing "light" colours on your screen, though, so don't expect a 100% match with the printed colours, but they will be closer to the latter than the original RGB colour.

Want to know more?

Wikipedia - Additive color theory
Wikipedia offers more information on additive colour mixing in this page.
Wikipedia - RGB color model
Wikipedia offers more information on the RGB model in this page.
PrintingForLess - Important information about RGB and CMYK
This article offers some more information on how to manage colours from RGB into CMYK.
Curvemeister
Curvemester offers some more information on how to convert RGB into CMYK as well as a wide gamut CMYK.
American Printer - Does painless RGB to CMYK conversion exist?
American Printer has an extensive article on solutions to convert RGB into CMYK.
Designer Info - RGB to CMYK: Gamut warning!
Designer Info has this extensive article on converting RGB into CMYK.
Forret color conversion tool
A handy tool for converting between colour gamuts.
Digital Expert - Color Space Fundamentals
More information about converting RGB into CMYK.