Rainbow Cake

For Molly's "rainbow party," the outside of the cake looked like this:

Before it was assembled the cakes looked like this (top two have been cooked and the bottom two are ready to go in the oven). I used these instructions, roughly making concentric circles... I just blobbed some batter in the middle, then each color on top of the last, jiggling the pan a little to help it settle. (If you click on that link, the lady gives some strange low-fat diet recipe; I just used regular white cake mix and followed the normal intructions.)
I just divided the batter into six small bowls and colored each with gel food coloring to make the six colors of the rainbow. This process made the cake less fluffy than usual, and after I'd baked two 8" rounds I knew I wanted it higher. So I used a second box of cake mix and baked two more 8" rounds. I split each round & put a thin layer of frosting between, so there ended up being 8 layers total. I used store-bought white frosting and the whole thing took 3 full cans of frosting. Also, the rainbow on top was made of m'n'ms, which I decided would taste better with cake than Skittles would.

I was just delighted when we cut into it; the colors inside were SO vibrant!
All the party guests ooh'ed and ahh'ed over it; here's a closeup of a single piece. I really like how it turned out.