The Mister's birthday is this week but due to a very full calendar, we decided to celebrate with family over the weekend. I made a rainbow cake which ended up being a big hit with the adults and the kids. When I said I brought a rainbow cake, everyone assumed in was a "Funfetti" cake and so were pleasantly surprised when we cut into it to reveal 8 layers of food coloring over dose!
I was sort of guessing as I went along and it seemed to turn out okay. Here is how I did it, for those of you who are curious: I used two white cake mixes and made them according to the package directions. Once the cake mix was made, I put one heaping cup full of batter into a bowl and added the appropriate food coloring. (It is important to measure the batter so all the layers end up being the same thickness. Also, I used Wilton's gel food coloring you find in the cake decorating section at Michael's. I think it is more concentrated than the liquid kind.) Since I only have two round cake pans, I baked two colors at a time, 350 degrees for 15 minutes, and then washed out the pans and reused them. I was totally guessing with the amount of batter for each layer and I ended up with one cup left over. Hence the pink layer in my Roy G. Biv cake. I frosted the cake with Swiss Buttercream. It's not as sweet as regular buttercream and I think it is easier to work with. You can find a recipe for it here. (Note: I used the recipe for a 9 inch cake and I had enough frosting for the whole thing. However, I only used a scant amount between each layer. If you like a lot of frosting, you might consider doubling the frosting recipe.)
(Inspiration for the rainbow cake from here.)