I just used a boxed cake mix and used a 1/4 cup measure to scoop batter into each bowl, until I ran out of batter (it is important that you have roughly the same amount of each color, or else your last few cupcakes will be purple, blue, green and yellow only ) You only put a tiny amount of each batter into the cupcake liners, and don't worry about spreading it out or anything, it all works out when they bake. Be careful not to overfill, because you can't take any batter out once you put it in! Also, food coloring gel works waaaaaaaaaaaaay better than liquid color, you need way less of it.
Once they were done, I let the cool completely, and since my frosting turned out to be yellow, not white like I thought, I dyed it blue, and then I used marshmallows to make the clouds. I microwaved them for about 10 seconds and then immediately put them on the cupcakes, in little clumps of 3. Without the microwaving, they just look like marshmallows, and if you wait too long, they start to get sticky and you end up having to scrape them off your fingers.
To make the rainbows, I got Airheads Extremes rainbow sour belts, at the Dollar Tree. Cut each strip in half, and gently pressed each end into the marshmallow clouds. Taa daa!
