You start by making a white cake mix. Like I said, I used a box - just followed the directions as printed. Then I used a 1 cup measuring cup and divided the batter into 5 disposable bowls. There was a little bit leftover but I just tossed it because it wasn't quite enough for another color. Using food coloring, dye each bowl to your desired color. I used the regular food coloring and it took 1/3 to 1/2 of each bottle to get them this bright.
Next, I took a 6 inch round cake pan and poured in about half of the blue, half the green, etc. This is where I think I went wrong. There was waaaaaaay too much batter in the pan and they took nearly 50 minutes each to bake. (I only had one pan)
Once the cake was cool, I transferred it to a cooling rack and started the second layer. This time I put red on bottom, then orange, etc.
The next day - after storing in plastic wrap in the fridge - I shaved off the top of the bottom layer, added a layer of icing and placed the second layer on top. I didn't shave that layer off because I liked the rounded top look. And again, as a result of putting too much batter in the pan, they puffed up super tall, and if I would have leveled it off I would have lost a lot of cake.
I am not a cake decorator. So I did the best I could with canned frosting, my mom, and an aerosol can of decorator icing.
I couldn't wait to cut into it to see if it had turned out. And.........it did. It was perfect!