A while back, I saw a post on my friend Adrea's blog about a rainbow cake made with boxed cake mix and gel food color. I clicked on the link she had included and made my way to the Omnomicon blog (as in "om nom nom"), where they had detailed instructions and some beautiful photos.
I just so happen to know a little girl who's really into girlie things and rainbows and such, and I thought "That doesn't look so hard. I should make that for her birthday." So I showed her the pictures and asked if she'd like a cake like that this year, and she jumped on it.
Basically, you make the cake mixes as directed, then divide them up evenly into 6 bowls. Into each bowl, you put a color of the rainbow, and stir it to get the color mixed evenly. Then you layer the colors into 2 round or 9x13 cake pans and bake. (Check the Omnomicon post for detailed instructions.)
It wasn't that difficult, really, but it took some time to get a few of the colors really deep and saturated. The red in particular took quite a bit of color to get it looking truly red and not just pink, and the purple took a healthy amount of gel paste, too. I think that mixing some of the colors a lot more than others made the cake rise unevenly, since the pans came out with off-center humps. So I think my goal would be to stir the cake mix just until evenly dampened, then try to stir the bowls a roughly equal amount when mixing in the colors.
I forgot to take any photos while the batter was still in bowls, which is a bummer, because it looked cool. But here are the layers in the pans, ready to go into the oven:
Close-up of the purple/blue/green layer:
The baked layers:
And here's a slice of the finished product: