This was a fun project! I had the kids make large, mishapen rings on watercolor paper, using a black crayon. Then, I showed them how to make different shades of the same color by lessening or adding water. The more water, the lighter the color, while having less water produced a stronger and darker color. They added a little bit of black, red, or yellow as they liked, and then filled in the empty space with black.
They turned out looking really cool!
The original idea can be found here.