When you do find Rainbow Ink around the map, you'll be able to unlock some of the pre-set styles that Epic has added into the game for Toon Fish. While on the flipside, finding Color Bottles, enables you to deck out the skin with whatever color combinations you want.
If you're looking to find Rainbow Ink, you'll need to find and loot Cosmic Chests around the Fortnite map. They're unique chests that spawn in every game mode besides solos, as it requires more than one person to open it. They're noticeable due to their otherworldly appearance, and will be hovering off the ground.
Player's will be able to earn a maximum amount of 25 Rainbow Ink each week, and over the course of the season, this is capped at 250. So, if you're up to date with all the ink, you should have all the additional styles unlocked at the tail-end of Season 8