Here is the tutorial for how I made the rainbow cake. Though it takes a lot of time, it really is an easy project. If you try it out, please tell me how it turned out for you!
2 White cake mixes
Wilton food dye (red, orange, yellow, green, blue, purple)
8 inch round cake pan
I used this white cake mix.
It needs to be a white mix so the colors come out true. I used two boxes and made them according to the directions on the back. I did one box of mix at a time, though.
After mixing the cake, I divided the batter into 6 bowls. My picture only shows three bowls because I mixed the second cake up while the first was baking.
After dividing the mix, I used Wilton food dye to color each divided mix.
These Wilton food dyes are gel-like so I just used a toothpick to scrape out what I wanted.
I mixed the batter and dye until I came to colors I was happy with.
I have 2 8-inch cake pans so I was able to bake two at a time. I had to watch the cakes closely so they didn't burn. They are much thinner than a regular cake so the times on the back of the box will not work for these. I ended up baking each layer for 15 minutes. I let the cake cool in the pans for another 15 minutes and then turned them out on a cooling rack. Don't forget to butter and flour your cake pans so the cakes come out easily!
I then wrapped the completed layers in cling wrap and froze them. I made the cakes on a Wednesday afternoon and then frosted it on a Friday afternoon. I had read that freezing the cake helped it to not be so crumbly, but I didn't think it really worked that well. Although, I did think freezing it made it easier to work with because it was harder.
When I was ready to frost, I covered a cardboard cake circle with foil and laid my first layer (purple) on it. I used an extra large frosting tip to cover the layer with frosting and then laid the second layer (blue) on top of that. I jiggled the layers back and forth with my hands until they were on there evenly. I was going to cut off the crowns, but decided to do it this way and I'm so glad because it worked just fine.
I continued stacking until all the layers were on in the order of the colors of the rainbow. Red, orange, yellow, green, blue, purple.
I then used the same icing tip to cover the outside and tried to smooth it as well as I could. I'm not a baker so I don't really know how to do this to make it as smooth as cakes that you buy.