You can make a lot of different kinds of rainbow food. We have tried: cupcakes, waffles, banana bread, muffins and cake. I think it would work with any thick batter?
Whatever you decide to do it's pretty simple.
1-Make your batter...when I've made these I simply follow the recipe...as long as the batter is somewhat thick and not super runny it should work.
2-Divide batter equally into 6 different cups (each cup will be for a separate color)
3- Add a couple drops of food coloring to each cup
Remember: Yellow + Red = Orange, Blue + Yellow = Green, and Red + Blue = Purple (When mixing colors use equal amount of each color)
4- Layer batter in tin/pan (In the picture below I started with red and then went down the line of cups)
5- Bake according to recipe directions
6- Cool, Frost if desired and Enjoy!
Note: If you are doing a cake it's really fun to cover the cake with frosting and so nobody knows it's a rainbow cake until you cut into it.