I ordered this color wheel ($9) to help with choosing colors for map layouts. Each color has the RGB value and the HTML code listed right on it. It only lists the 216 "web safe" colors. Even though I am not specifically designing for the web, it will be a good idea to limit myself to these colors because the maps can end on the web.

I bought this pocket color wheel ($3) from the Purdue University bookstore when we stopped on the way back from Indianapolis. It has been helpful, but I still have to go and look up the RGB values of the colors I want to use, and it only has 12 colors.