The mentalist shows a black envelope and states that inside it there are four kings. He asks a spectator to think of one of the kings and name it. Then the mentalist takes the cards out from the envelope, he fans them and mysteriously, the thought of king will be the only one turned face up among the others. But that's not all... the thought of king is the only one with a different colored back.
Very easy to do!
The cards are Bicycle standard poker size.