This graphic is an optical illusion that demonstrates how the human eye adapts to the perception of colour on different backgrounds. The pink square appears darker on the light grey background and lighter on the dark grey background. However, both pink squares are identical in colour. The perception of the intensity of the pink square is manipulated in the brain and does not result from the physical properties of the light. The brain creates a simultaneous contrast in which it adjusts the perceived properties of the colours depending on the brightness of the background.