I made these rainbow cupcakes for the Young Women New Beginnings program this past weekend. I cheated and used a boxed cake mix, so this is more tutorial than it is recipe.
I recommend using plain white cupcake liners, instead of colored ones.
Cake mix, eggs, oil, and water as directed. Mix well.
Divide the cake batter equally into six different bowls. Get a separate spoon for each bowl. Add lots of food coloring. I used a lemon-flavored cake mix, so it was yellow to start with. My most difficult colors, the ones that required more drops than others, were the red and blue. I finally got good shades though. Don't be afraid to use a lot of food coloring to get the depth of color you really want.
Once your colors are shaded to your satisfaction, decide if you want to start at the top or bottom of the rainbow. I meant to do purple first, but spaced and did red first. It doesn't really matter much, though. Spoon all of one color into the bottoms of your cup liners. Repeat with the remaining colors. The layers won't really stay in even layers. They'll each squish each other a little farther down and cause a little more spreading to the outside, but all unevenly.
Bake according to package directions. (If you don't have enough batter to fill all the cups in a pan, fill the empty cups about 1/3 full of water, to help heat things more evenly.) When they come out of the oven, turn them out onto a cooling rack.
They'll be pretty, but maybe not exactly what you were expecting. That's okay. Let them cool completely.
I used Joy the Baker's recipe for brown sugar cream cheese frosting. I halved the recipe and still had a lot leftover. I didn't pipe the frosting on, though -- nothing fancy. I just spread it on and then added some colored (and white) sprinkles. The cupcakes look very unassuming.
Even unwrapped, they aren't very exciting yet...
Until you open them up. Then, it's a party on your plate, and in your mouth!