This one was fun to do. The original idea was to use water colors that would be kept in by the black crayon lines. My water colors come in tubes and there was no primary red so I opted for watered down acrylics. I enjoyed using acrylics again, but I think the water colors would be better for this since they wouldn't leave such bold brush strokes.
This would be a fun one to scan in and play with digitally. Here's the original idea on Art Projects for kids. Her instruction for making the boxes were very helpful!