Doing these reminded me of some some color cards I made a while back, as suggested by Nita Leland in her book. I started off with the usual color schemes, complementary , triads, tetrads etc., and keep adding to them whenever I find something new. Notes can be written at the back, of the colors used etc. Its a useful and fun exercise, it makes you think of hue/value combinations and keeps your eye sensitive to colors outside. And of course, serve as references for future projects and paintings.