One of the reasons I love Phil Plait's Bad Astronomy website is because not only is he a brilliant astrnomer and movie buff, but he's a total geek and nerd of the highest order, which makes him OK in my book.
So, from his blog, please regard the image below:
I love optical illusions... in this image, you can see three embedded spirals... one is green, one is pinkish-orange, and one is blue... right? Wrong! In fact, the green and blue spirals are actually the same exact color! How cool is that? Don't believe me? Check out Phil's full entry for the complete explanation... it's worth it, trust me.