Rainbow Diamonds are the premium currency in ONE PIECE: Bounty Rush, used for summoning characters, especially powerful ones like those from the "Extreme Legendary Bounty Festival," and for other in-game purchases. They can be obtained through login bonuses, missions, special events, and can be purchased with real money. It is important to note that Rainbow Diamonds are platform-specific, meaning they cannot be transferred between a mobile device and the PC (Steam) version of the game.