Stare at the plus sign on the left for about 30 seconds. As you do this you probably will see some colors around the blue and green circles. After about 30 seconds, shift your gaze to the plus sign on the right. What did you see?
You probably saw a yellow and desaturated reddish circle. Did you notice that they were not there before you tried this experiment? Furthermore, unlike the colored circles on the left, the ones you saw on the right moved around as your eyes moved, which proves that these perceptions are the result of afterimages in your visual system and not some trick of the stimulus presentation on the right.
It is observations such as this that led Ewald Hering to propose the "opponent processes theory"