William pointed me to this article applying Josef Albers’ color theory to UI design, although as usual UI designers seem to think what they do visually is somehow different from regular old graphic design (which it’s not). In any case, I highly recommend Albers’ book Interaction of Color, though the original has about 10 times as many color plates. Also, I didn’t realize there’s a CD-ROM version, which sounds interesting. You’ll need Mac System 7 and 2 MB of RAM (oh, and $145).
In case you use a PC or don’t feel like spending $250 on a fancy color wheel, I spent a couple hours one morning at the beach last week and made a little web-safe color sketcher grid thingy. If I get a few minutes I’ll add a way to define non-web-safe colors.
PS—it’s MSIE 5 (maybe 4) only because it was quicker and easier that way and it’s just a little developer toy for me and mine, so don’t go reporting me to the standards police.