Jacob Barfield - Thief Protection

You have two packets of cards. One packet is red and the other is black. You place the red packet into a bag, and close the bag. You say your need to stop me from stealing the red cards from the bag and replacing them with the black cards. The magician shows all the cards in his hand are black. He takes off the top card, and in a split second it changes red. Not only does that card change red, but the whole packet of cards changes red as well. And the cards in the bag change into the black cards.