Honestly, store bought frosting works great in this. You don't have to be all fancy and make royal icing. Plus, I'm not actually sure how well the rainbow candy would be in the royal icing. So go with the store bought tub. Now, I know. Store bought frosting is a little bit lackluster. That's why you whip it with a hand mixer before using it. It'll add air into the frosting, which gives it a more appealing texture. Extra bonus: It also causes the frosting to expand, which means more frosting for the same amount of money. That's definitely a frugal tip.