This promo adds three grey Joker bricks to Firenze.
At the beginning of the game, place them into the bag with the other bricks. A Joker can be used in place of a brick of another color. You cannot start a Tower with a Joker, but you may use one to extend a Tower of any color. Joker bricks may not be acquired by trading in phase 2, but they can be acquired when you buy a card with a Joker brick on it. The Alchemist card may be used to take a Joker brick from the bag.
For more information, please check the BGG listing.