Polymer Clay artists are all about color and color tricks. Here’s a site that’s all about optical illusions and visual phenomena.
For this illusion, 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.
There really is no green dot, and the pink ones really don’t disappear. We don’t always see what we think we see.