Irooni (色鬼, Colorful Demon) is a technique used by Shunsui Kyōraku's Zanpakutō, Katen Kyōkotsu.
This is a color-based game, where you say the name of the color you want to cut with your sword and you cannot cut anything else. If the color you call does not exist on your body, the damage is minimal, even if the attack would normally inflict a much more severe wound. The opposite is true as well; by calling out a color you are wearing and attacking, the damage given is proportional to the amount of that color on you. To maximize damage, you have to use a color which maximizes the risks to your self as well as your enemy's. For example, by wearing black robes and calling "black," one would inflict massive damage on an enemy if struck, even if the actual blow itself was minimal or minor, but the caller would also suffer serious damage if he/she is struck by the enemy in return.