Here's the pictorial how-to, if you are so inclined to give this a try:
First, find your favorite cake mix. I'm sure you could use a box version, but Felicia chose to go it from scratch. From Betty Crocker:
Quick Yellow Cake
makes one 9x13 or two 9-inch cakes
1/2 cup butter, softened
1 1/2 cups sugar
2 1/4 cups flour
1 teaspoon salt
3 1/2 teaspoons baking powder
1 1/4 cups milk
1 teaspoon vanilla
Take the butter out of the fridge to soften and preheat the oven to 350°F. Prepare one 9x13 pan or two 9-inch round pans by greasing them thoroughly with butter or non-hydrogenated vegetable shortening. You can also flour them, if you want, although this isn't strictly necessary. Sprinkle a little flour over them, tilt and shake to distribute evenly, then tap out the excess over the sink.
Mix the ingredients together in the order they're listed - creaming the softened butter and sugar first, then adding the eggs, flour, salt, baking powder, and finally the liquids. Using an electric beater, beat everything together on low for 30 seconds, then high for 3 minutes.
Once you have your colors mixed, take a ziploc sandwich bag and fill the bag with the batter - one color per bag.
Once you have all of your colors added to your cupcakes/cake, you are ready to bake.
(CAKE INSTRUCTIONS ONLY) Let cool on wire racks for at least 15 minutes, then flip each pan over onto the rack and tap gently all over. Lift the pan slightly. If the cake doesn't feel like it's falling out smoothly, lay a slightly damp kitchen towel over the pan and tap again. If necessary, let the cakes cool more. If they have been baked thoroughly, however, they should fall right out of the pans once they've cooled a little and the sides of the cake have shrunk back from the pan.
After the cupcakes cool, frost/decorate as desired. Felicia experimented with frosting, and she chose to top these off with some butter cream frosting with a few sprinkles on top.