This would be very easy for kids to participate in. Just buy white carnations and add food coloring to the water in the tubes and watch the flowers change color. this would be very easy for high school and middle school kids.
Dilly-Dali Art: Rainbow "Pour Painted" Pots - easy and fun. This is totally designed as a kid's project but I imagine it could get really awesome if we just swapped out the rainbow colours for a more sophisticated pallette. Fast to make too!
Fun with shadow puppets! You can do these with kids of all ages. I did them with two and three year olds. While the cognitive ability to figure out the idea of the shadow, etc, was very different, everyone still had fun doing them.