DIY Rainbow Petal Cake by The Cake Blog. With another straightforward and totally achievable DIY. Kate Wagner of The Greedy Baker is using everyday items to decorate a spectacular rainbow cake, perfect for any celebration. It’s vivid, bright and has us cheering for a supply list including zip lock bags and spoons. And once you see how easy it is, you’ll be cheering too! It’s a DIY Rainbow Petal Cake!