Four double-blank cards are displayed. One is placed in view to one side while the other three are arranged in a column on the table. Four keys are shown, there being one each of red, blue, green and silver. A spectator selects one of the keys (let's say the green one) and it is dropped on top of the blank card just placed to one side. The other three keys are slipped into envelopes which are mixed by a spectator before being placed down next to the blank cards in the order they were mixed into.
The performer makes a magical gesture over the cards and on turning them over they are seen to be printed with pictures of red, blue and silver coloured padlocks. That's a surprise, but then when the keys are tipped from the mixed envelopes, the correct coloured key is seen to have been magically aligned with the same coloured padlock!
The final kicker comes when the card originally placed aside is turned over to reveal it has a green door printed on it to match the green key originally selected by the spectator.