You introduce a small packet of grey-backed cards with odd green and red dots in patterns that form numbers. You explain that this is the standard test to check people for red/green color blindness. If they can see the numbers in the patterns made up of the dots, then they are not color blind. On the other hand, if they can’t see the numbers, they should have their eyes checked. Upon confirming that everyone sees the numbers, you make a magical gesture and change the way your audience perceives colors. When you display the cards again, the audience is momentarily confused because everything looks the same. That’s when you turn the cards over, and the grey backs have transformed into a rainbow of colors. The cards can then be handed out for immediate examination.