This page has been updated to Update #56.
Rainbow Cookies Are a Premium Currency that can be used on: Buying Milkshakes, Buying Super Cookies/Super Structures, Buying Gold Cookies/Gold Structures, Upgrading the Bakeries level up speed, Upgrade Cupcakes boost, Buying a Time Warp, Upgrading the Badges Speed & Amount that can be on-screen, and lastly from Buying Light Cookies.
How to earn them: Edit
Buying Rainbow Cookie Packs with REAL MONEY.
Finding and opening Loot Bags.
Leveling up your Bakeries.
Rainbow Cookie Frenzy - happens every 8 minutes and 4 with Premium.
Earning Your Leagues Rewards.
Typing in Secret Codes.
Doing Daily Quests tasks.
Getting the free 5,000 Rainbow Cookie Pack.
Lastly from Season Rewards.
Rainbow Cookies used to have a much older look in the game, look at the difference, can you find it?