This was from a page on New Scientist that appears not to be online anymore.

I was taught at school that the three primary colours are red, blue and yellow and that by mixing these, all the other colours could be made. If you look closely at a television screen, you can see that the picture is made up of red, blue and green dots. Why and how do televisions use these colours and how, for instance, do they make yellow from them?

John Hawkins,
University of Canterbury,
Christchurch, New Zealand

Televisions start with a black screen and add coloured light. Painters start with white paper and add paint which serves to remove colours from the reflected light. The television uses additive colours, so called because adding more makes the image brighter. Painting uses subtractive colours and works in the opposite way.

As you might expect, the two systems complement each other. Adding yellow and blue paints together gives green, which is an additive primary colour. Conversely, yellow light can be made out of red and green light.

Here's where the fun starts. The subtractive primaries are technically yellow, cyan (an intense light blue similar to sky blue) and magenta (a kind of purple). True red paint can be made from yellow and magenta paint. The problem is that it is difficult to get a really good magenta, and so artists end up using red instead of magenta. Although this precludes being able to mix a good, clean magenta, it is rare to find one in the environment anyway, so it's not missed. However, without magenta, it is difficult to make a good, deep blue from cyan and so ultramarine is used instead. The effect of this is that green is now difficult to create, and so bright green is used. So really, although artists claim to use yellow, blue and red, they in fact use a couple of reds, yellow, green, blue, a large tube of white, black, and often an assortment of tertiaries. Painting is an engineered solution.

Gavin Peacock,
Brighton, East Sussex