A TOUCH OF GREEN ILLUSION

From jerrypournelle.com:
First an interesting illusion sent my Mr. St. Onge:
Subject: Damn cool optical illusion. Lots of limiting things about our senses, eh?
  If your eyes follow the movement of the rotating pink dot, you will only see one color, pink.
  If you stare at the black + in the center, the moving dot turns to green.
  Now, concentrate on the black + in the center of the picture. After a short period of time, all the pink dots will slowly disappear, and you will only see a green dot rotating if you're lucky!