OK,
here are a couple:

1. have them pick an old drawing (from prev. class), then have them colour it by:
a. one colour, hues of white and/or black.
b. primaries, secondary, or tertiary colours using complimentary for hues.
2. old drawing again, paint not using more than 3 colours. (this one drove me crazy untill Ifigured it out)

Well you get the idea, run with it. Best of luck.