This illusion is guaranteed to force an afterimage for most viewers.

Stare at the center of this illusion until you see a yellow-green dot following the purple dots. The yellow-green dot does not exist. It is an afterimage of the purple dots, and only exists as long as you stare at the center.

As you continue to gaze the purple dots will begin to fade away, but the the yellow-green dot will remain strong.

Try different colors using the color picker buttons. Each will force a different afterimage color.