Here is a project idea I stole from Kids Artist, and I absolutley love it! It was an easy project for all ages to experience hands on what happens when you mix blue and yellow. I loved the results, and had considered asking to keep 1 or 2 to hang in my house, then decided to just tell them to have their parents frame them:-)
ps- my 3rd. 4th, and 5th graders loved it, and it took about two 45 minute classes to complete.