This will drive you nuts! Make sure you read under the illusion, too.

If your eyes follow the movement of the rotating pink dot, you will see one color, pink.

However. 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, all the pink dots will slowly disappear, and you will only see a green dot rotating.

It's amazing how our brain works. There really is no green dot, and the pink ones really don't disappear. This should be proof enough, we don't always see what we think we see.