This is a work I normally have out from now (March) until the rest of the year. During spring its' fun to add a white bottle of paint (in addition to the primary colors) to make spring colored pastels!
Here's the colors all mixed up
Stirring them with a toothpick seems to work the best. When stirred with a paintbrush the paint typically gets all stuck on the brush not leaving a lot left to paint with. The small toothpick helps keep it neat as well. There is a wet sponge on the tray that they can wipe the toothpick off between colors so they just need one.
Then they make whatever design they please!