I found this similar idea in a google search just now. Interesting how different people can come up with identical ideas. The exact inspiration for me likely came from several sources.
The directions are simple. Draw a variety of shapes using the technique of overlapping. I distinguish between three types of shapes: Geometric; Organic (that found in nature); and Free-form. Some students needed to be "encouraged" to add more shapes to their pictures to balance them out. When the drawing (done in pencil) is done, it can be outlined in a black marker. If you're brave, have students just draw in a permanent black marker as it saves a step.
You can use any kind of color scheme you'd like (for example warm colors for shapes and a cool color background). Or just let the kids decide. One rule I had was that they had to use a variety of colors and that two connecting shapes could not be the same color. Each section of the shapes had to be colored in different colors. (see the examples to get an idea).
For coloring we used markers and I teach my students to treat the marker like it's a paint brush. The tip of the marker is like the tip of a brush. You can make thick and thin lines depending on how you hold the marker.
All lines in a given area are to follow the same direction. That means that each section is treated like a canvas unto itself. Circle and oval shapes can have circular lines but straight line shapes must have straight marker lines (and they must all follow the same direction. Students may first outline the shape but then all subsequent lines must have the same direction. The outlining helps students to keep withing the boundaries of the shape. I have my students point their marker tips toward the black border line when outlining the shape.
I think the effect is rather nice. Look closely at the lines in each example. You'll see what I mean. If this doesn't make sense, please let me know. I'll try to clarify.
However, you will likely see where some students didn't exactly follow the "rules" above. You can decide for yourself if this matters. Let me know what you think. It's a great lesson for teaching color theory, balance, overlapping, and technique. Here's a gallery of work.