If your eyes follow the movement of the rotating pink dot, the dots will remain only one color, pink.
However if you stare at the black " +" in the centre, the moving dot turns to green.
Now, concentrate on the black " + " in the centre of the picture. After a short period, all the pink dots will slowly disappear, and you will only see only a single green dot rotating.
This and many other illusions courtesy of Michael Bach of the Universitäts-Augenklinik Freiburg.