Fruity Rainbow Flakes are a cereal Phineas & Ferb eat for breakfast. They are seen eating it in the beginning of the episode "Hail Doofania!". The cereal also powers the rainbow-inator, made by Phineas and Ferb. (Phineas and Ferb eating Fruity Rainbow Flakes) Isabella: What'cha eatin'? Phineas: Fruity Rainbow Flakes, you want some? Isabella: Oh no thanks, I already had breakfast.