Look at the chess pieces below. Most people see a light set of pieces and a dark set of pieces on differently shaded backgrounds. This is a perceptual illusion and your brain is tricking you. Your mind can only see the picture as a whole – taking into account the background colors as well as the chess piece colors. Then your mind goes back to what it historically knows about chess pieces. We’ve all seen light and dark chess pieces before, and our mind jumps to the conclusion that’s what it sees here. However, if you use the slider and change the background to a solid grey color, you will see that the chess pieces (top and bottom) are the exact same colors.
Now you mind doesn’t have to account for the background shading and it doesn’t cause the perceptual illusion any longer.