A deck of Hoyle cards is shown to have blue backs. The deck is then turned face up and fan spread. Any card is selected by the spectator (truely a free choice).
 
The magician states that one of the cards in the deck is marked on the back. The spectator's card is turned over and it's back is seen to be yellow.
 
Next the yellow card is placed on top of the now face down deck and shown to have changed all the backs of the cards to yellow. When the yellow card is removed the card backs are blue again.
 
For a finale, the deck is again turned face up and the bottom blue back card is shown both sides and placed face up on the face up deck. Then the previously selected yellow backed card is placed face up on top of the blue backed one.
 
The cards are "mixed" and the spectator is asked what color is made when blue and yellow are mixed. Green of course. The deck is then turned over to show by ribbon spreading it that all the card backs have turned to green.
 
This is a very striking visual effect.
The trick deck can then be switched off for a regular Hoyle green backed deck ($7.00) and any other card tricks you like can be done as a follow up. Very entertianing crowd pleaser....!