I stole the following idea from a fellow preschool teacher ;) My 4 year old came home with these last week and I thought that they were so great that I had to share. The best part was that when I asked her what she learned at school that day, she responded, "Well, I mixed blue paint with red paint and something happened, Mom...it turned the paint purple!
Simply place two different paint colors on a sheet of paper and have your child swirl them together (with their hand, paintbrush, spoon, etc.) until they find the new color. You can do blue + yellow, red + blue, red + yellow, red + white, or any other variety that would make a fun new color.
As an added learning adventure, have your child "guess" what color will come from each mixture ahead of time, and then have them compare the results. What a fun, easy, and frugal idea to help children learn about colors.