Mirror image

Start by allowing the spectator to shuffle the deck with a riffle shuffle. They can cut the deck too if they want.

Now show how the deck is shuffled to the spectator. You have to find where 2 cards next to each other are of the same color : you must cut there, between the 2.

The spectator can now do the rest of the trick. First they flip the top card, and take it with the next card face-down, and create 2 piles of red and black cards. They can repeat this for the whole deck.

Finally you can spread the 2 packets, flip it, and show that the spectator has successfully sorted all face-down cards.