The magician shows a normal red deck. You pick a card. He does 'something magical', and suddenly the whole deck is blue, only your card still red. This may be the most spectacular card-trick finale in the world.
Watch this trick in action
The technique: gimmicked deck
The secret is a specially manufactured deck. One version has all cards with a red back on one side and blue on the other, flipping the deck swaps the colour. Another version uses a 'double-backed' deck consisting of cards with two backs of different colours.
The most powerful version combines techniques: the deck is first shown as 'normal' (all cards red), and after a quick Elmsley count or a Stripper cut the colour is visibly swapped. The chosen card stays red because it was kept apart before the switch.
The psychology: colour is a 'cognitive landmark'
Research shows colour is one of the strongest recognition markers in the brain, stronger than shape or number. The brain registers a colour change instantly and powerfully: this cannot happen without cause. That's why this trick feels 'bigger' than other card tricks.
The effect is amplified by one card (yours) not changing. This creates what psychologists call 'differential salience': the contrast accentuates both sides. The switch feels personal, as if the magic exists especially for you.
A gimmicked deck and a good story. With that, 30 seconds of magic look like a world wonder.