There are so many ways to choose colours, for packaging or websites or whatever. The Red Queen Color Theory covers a few [via eatonweb].

Previous to designing (this revision of) this 'site, I'd been locked into the idea of colour "chords" (as I called them) or triads for a long time; it was quite constraining, but I couldn't think of any other ways to choose. Simply put, it's two similar colours and a constrasting one which "work" together. Once I had the idea in my head I couldn't shift it. When it came to redesigning this page, everyone was using blue, orange and grey and I wanted to move away from that. I started with obnoxious colours, brown and red mainly, and worked from there. It's much more of a challenge to take horrible colours and make something that looks okay. I think it kinda worked (many people really hate the scheme, but that's a good thing. I don't want to please everyone), but I've still not managed to completely escape the triad concept.