This is a very useful page about color theory from Write Design Online: http://www.writedesignonline.com/resources/design/rules/color.html
While working at Bead Obsessions, the help I was asked for most often was for help choosing colors. I usually said things like "Choose colors that make you feel good," or "Listen to your gut." That's what I usually do. I make a pile of beads (or yarn if it's a knitting project), look at it, watch it in different lights or as the light changes, and gauge how I feel about it.
Or I might use things in art and nature as inspiration for color choices. The link above uses art as examples to explain the terms used in color theory.
