The following demonstration is a great use of the color after-effect illusion. The image below will give an example of a color you will never see on your computer screen. Stare at the dot in the centre of the red circle for three to five minutes.

You will see the edges of the circle begin to glow, but do not stop staring at that point - keep on. After a few minutes move your head back and you will see a glowing ring around the red circle. The color you are seeing is a cyan which will be perceptually totally different from any of the colors you have seen on your computer.

The color gradient to the right shows the shades of 'cyan' which your computer can produce, none of which match the color you see with this effect.