A mate of mine suggested a projector last night. That wouldn't work, as he turned the balls around and also prodded them.
So that got me thinking... ever heard of e-ink? It's what they use for the new fangled book readers. Charge some thingy-ma-bobs the right way and they 'stick' to the surface and stay there until charged again.
E Ink - Wikipedia, the free encyclopedia
I'm sure you could use that technology in a ping-pong ball - after all, the e-book readers you can buy are wafer thin.