Steven Universe: Unleash the Light

You may have noticed while playing that charms come in different colored cubes, which dictate how common they are.  For the vast majority of the game, you will only see white, blue, or purple cubes - denoting common, uncommon, and rare.  You won't encounter yellow (legendary) until you start playing the Black Hole dungeon on hard difficulty.  Rainbow charms are also in the Black Hole dungeon, on diamond difficulty.

You may randomly receive one from a treasure chest, or be able to buy one from a shop.  But you are guaranteed to get one when you beat the diamond difficulty boss, so this trophy effectively goes hand in hand with Space Train to the Cosmos .