If you were lucky enough as a kid to have a Lite-Brite set you know how mesmerizing and fun it was. If not go buy one and reclaim some of your youth for just $16 here. If you are rich, go ahead and skip that last suggestion and head over to Hero Design for one of their Everbright displays.
The Everbright display has dials that can be turned to show just about any color you can imagine. They are huge and have price tags to match.