I saw this on several blogs and thought I'd give it a try. We used a bit of plain unflavored yogurt and food coloring and went to work on our paper plates. It was really fun! The boys love plain yogurt, so they did a lot of licking. I intended to have them work on forming some letters with their 'finger paint' to work on how a letter is made. After a couple frustrating encounters with my firstborn, I just let them have fun.
Of course, the little scientist he is, John Paul mixed the colors together gleefully and told me all about what new colors were appearing. Easy to make, easy clean up, and good pre-writing practice (in theory).