If you're working in pastels, or an other mixable colour medium for that matter, and like me have a very limited number of colours here's a cool tool I've found which might be useful.

Scroll down the page to the Interactive Colour Chart. Well, it's not really interactive, at least it doesn't work that way for me, but it still seems to be very useful. The site actually suggests you should work with no more than 10 colours, even as little as 4 primaries, so as to train yourself in colour mixing.

That is nice.

Here I are some pictures I took from Ask Jeeves of the Color Wheel Chart, should also help others for complementary colors and what colors make what.