From David Pogue's New York Times blog comes this:
Pogues Posts: A Wacky Persistence of Vision Test
New York Times, June 10, 2005
By DAVID POGUE
This wild and wacky persistence-of-vision test is a new one on me. There is absolutely, positively no green in this circle of dots--but even I, Mr. Color-blind, saw phantom green spots after only about three seconds.
[swb note: there are many more optical illusions at this site, including face perception & emotion and a cool "stepping feet" illusion]