The spectator selects a card, which you fold and both of you sign. Tear the card to remove a corner. Now, in a blink of an eye, the corner changes color! From BLUE to RED or from RED to BLUE, the almost automatic gimmick will allow you to do the 2 versions!
But this is not the end! With the signature always visible, you place the different-colored corner on the card, and you reattach it! You can now give the card to the spectator as a souvenir, and their signature will be on the reattached corner. INCREDIBLE!