Truly Astounding Effect!

Show a blue-backed deck and ask a spectator to select a card, but not from the deck.
Have her select the card from your very magic tie, which you have on.
Okay, they play along - the spectator thinks of one of the cards as you spread the deck face up. Ask her to name the card she was thinking of. 
You remove the chosen card and place it face up in front of the spectator. 
Re-spread the cards once more to show them all blue. 
Now for the big climax -
Turn their card face down to SHOW THAT IT WAS THE ONLY RED CARD IN THE DECK!
Card on Tie comes complete with deck, tie and complete instructions.