Andrew Rapp made this neat application to help you quickly find appropriate colors to use for your LED project. He hooked up an RGB LED (that is, a single LED module that has red, green, and blue LEDs built into it) to a color picker dialog box on his computer, allowing him to change the color of his light by clicking on his screen.

If you are like me and have never really studied color theory, Geoffrey Milburn has a nice explanation about how additive color mixing works.