Some challenges I've seen or participated in, a crayola challenge, an M&Ms challenge, Fire and Ice challenge, My State My State, my favorite book, what can you do with a panel, ugly fabric (everybody had a fat quarter of the same fabric), and one challenge where everybody drew a paper item out of a bag and then had to use the same colors in the same proportion. The paper things were greeting cards, food labels, paper napkins and that sort of thing.
I probably wouldn't participate in a color challenge based on the letter my name starts with.