Here's an idea I was kicking around for a while:
Start with a red deck. In your pocket you have a blue card from another deck - say the 3C, the red double backer and another card from the blue deck.
Have the red deck shuffled, take it back and bring out all three cards squared together, plopping them on top of the deck and immediately thumbing off the top (blue) card. A red back will show and everything looks normal. You can if you like emphasise the situation by spreading through the deck being cardful not to reveal the blue second from top. Calling the blue card a prediction you place it somewhere on your person - tucked into your t shirt sleeve, in a shirt or jacket pocket poking out etc.
Now you need to force the blue card - a great method for doing this is a marlo move usually used for sandwich effects based on the bluff pass, which not only looks very fair but keeps everything in position. I read about it in Mike Powers "Power Plays" book. Anyway, at this stage a card has been selected (the idea of a force should be minimised by the fact that you have not looked through or rearranged in any way the deck they shuffled).
Get a break under the top two cards, take your blue card and put it back on top of the deck, do a triple lift and thumb off the single card, showing it to be blue on one side and so demonstrating the prediction. A red card shows underneath thanks to the double backer. To clean up get a break under the top two cards again, place the "prediction" card face up back onto the deck, pick up the triple casually flashing the blue back and place it back into the pocket (if you put it back the same way you had it to strt with you're now reset).
Some additional thoughts:
don't be afraid of the triple - its only out there for a moment and its constantly in motion. The fact that you drop it onto the deck loosely and instantly thumb off a single also helps to sell the "singleness" of the packet. This principle is a great technique for adding and subtracting special cards to and from your deck - so long as you can find a compelling reason to add a card in the first place! I use it a lot - the simple act of bringing out an "unusual" card helps to engage your audience, creates visual interest and anticipation of something a little different so its a great addition to your card magic.
The audience never sees the face of the first blue card in the effect as described above. This means you can potentially have some fun with it! Using a 52 on 1 card for example would give you a funny ending - you predict the card they chose to start then change it to the 52 on 1 card with a line like "and if you'd have chosen a different card, well, I had it covered!". Another option would be to use a red/blue double backer for the first blue card so that after you reveal the successful prediction you can spread the red deck very freely and only red backs will show.
Another thought would be to have the chosen card signed (you would be using some kind of bottom-slip force to have the card selected without revealing the back and have it signed as well). The first blue card is a duplicate. Have them name their card and turn over the first blue card to show the succesful prediction. Then use a change - e.g. shapeshifter with the top three cards to name but one of several options - to have the signiature appear on the card, which again you can instantly thumb off showing only red cards. Think about that effect for a moment - THEY shuffle, you don't rearrange the cards at all. A "single" blue card is introduced and placed aside and they choose a card - forced yes, but a strong force because of the shuffle - and sign it. You then reveal that the blue card matches their selection and have their signiature appear on the card to finish - all made possible by the double backer! Clean up is identical.
These are just some random thoughts - nothing fully developed yet, but enoguh to get your wheels turning perhaps. Another place to look is Jack Parker's 52 Memories, a great collection of card magic including a three phase routine using a double backer that has some very clever uses for the gaff.
Cheers,
David.
