The bonuses a player has from development cards acquired on previous turns provide discounts on the purchase of new cards. Each bonus of a given color is equal to a token of that Color.

Thus, if a player has 2 blue bonuses and wants to purchase a card which costs 3 blue tokens, 3 red tokens and 3 black tokens, the player must only spend 1 green token.

If a player has enough development cards (and therefore bonuses), they can even purchase a card without spending any tokens.