To kick off painting in my middle school classes this year, grades 6, 7, and 8 all did an exercise where I challenged them to mix 64 different colors using only primaries (red, yellow, blue) and white paint. I gave them a grid with 64 pre-printed squares (I just made it in Microsoft Word by using the table maker) on 11x14" copy paper. We didn't bother with water and instead blotted brushes on dry paper towels between colors. The paper towel method has been a favorite of mine this year...in a room with no sink it really saves time now that I don't have to fill water containers on painting days! Another perk: my students have much more sophisticated colors due to the inevitable color mixing!