I made these for my son Donovan's 1st birthday party and they turned out phenomenal!
1 white cake mix
1 C sour cream
1/2 C milk
1/3 C vegetable oil
Combine all ingredients until incorporated (about 30 seconds). Scrape sides of bowl and then beat on med-high speed for 3 minutes. Divide batter according to how many colors you are using. I used 5 colors and it worked out to a little over 1 cup for each color. Use food coloring to color batter to desired intensity. I used approx 1-2 tsp. and they were very bright! You could easily make pastel rainbows using a little less.
To get a layered stripe like I used, you'll want to put a spoonful of each color in the cupcake one at a time. The batter is thick, so it won't spread on it's own. A great tip is to set out a little bowl of water where you're working. Dip your finger in the water and then gently spread the batter out. The water will make it so the batter doesn't stick to your fingers. Another tip is to use a measuring spoon and some water to measure out how many teaspoons/tablespoons are going to fit in your cupcake. Then divide that by how many colors you have so you know how much of each color to use for each cupcake. For my 5 colors, it worked out perfectly with 2 t of each color in each cupcake. So I put in 2 t of blue in the bottom of each cupcake and spread it out with my finger. Then I put in 2 t of green on top of that, then yellow, etc. Bake them according to the package directions, until a toothpick comes out dry.
Check these babies out! True crazy colors. I love how you can see the rainbow through the papers. If you want blue on top, just reverse the order (duh, right?)
A double batch made enough to put crazy high tops like this on all 24 cupcakes.
This is the frosting I used, here. The recipe is from a bakery in New York City. I thought it was amazing!