The Eyes Don't Have It

The second set of squares only seem coloured if you stare at the coloured squares first.

Your eyes contain colour detectors - each sensitive to red, green or blue. If you look at a grey square, all your colour detectors are working equally and balance each other.

But if you stare at a colour - for example, red - your red colour detectors soon get tired.

When you switch to looking at grey, your other colour detectors go back to work, but the tired red detectors are less sensitive and don't balance the others. The result? The grey square looks cyan - a combination of green and blue and the opposite of red.

How long does this false colour or afterimage last?

If you tried Eye Test 2, you'll know that it takes only a few moments for tired detectors to recover and for the afterimage to fade to grey.