Here's a simple, but fun exercise I do with younger students to introduce color theory. Thought I'd post since the hearts are appropriate for Valentine's Day, however, you can choose any simple shape or object you'd like. I've also done these in the spring with diamond shapes and had the students make them into kites. You could also tie this lesson into learning about Jim Dine.
These hearts were painted free hand, but I've done this project before with younger students and had heart shaped templates cut out of cardboard so the students could trace them.
These where done with acrylic paint on an 11 x 14 stretched canvas and sealed with a high gloss varnish. This could also be done on paper with other types of mediums.
As you can see, the canvas is divided in four sections. We started with the three primary colors, and then mixed our secondary colors. One square is warm colors; one is cool colors; one is primary and the last is secondary. The black outline really ties it all togethers, hides messy edges and makes it "pop".