An eye test is given to a spectator to see if he is color-blind. You go through a deck of cards showing him the fronts and backs. Of course the spectator tells you that the red cards are red, the black cards are black and that all the backs are red. The spectator freely selects a face up card.
He is shown that he has picked the only red card from a blue pack. The question is asked, “How can you play cards with one red card in a blue deck?” A rub on the table and that card also changes to RED! Remember, you started with an entire BLUE deck and end up with an entire RED deck! You can now do any other card trick that can be done with a regular pack of cards.