Line cupcake tin with 24 liners.
Mix cake mix according to directions on box using only the egg whites. Divide batter into 4 bowls(almost 1C batter per bowl). Tint each bowl with a color. I usually put 15 drops, but it's totally your call.
Start by placing a scant teaspoon of batter(same color) into the liners. Then move to the next color. That's to make sure you get all 4 colors in 24 cupcakes. Then divide the rest of the batter-1 teaspoon at a time- till bowls are empty. You don't need to "swirl" colors together. They turn out great without.
Bake according to cupcake directions. Let cool completely & frost. Enjoy!
I didn't have colored sprinkles, but I did have tiny White Nonpareils (WalMart,Hobby Lobby, Michaels). I poured about 2Tbs into 4 snack size baggies. I dropped 1 drop of neon color in each bag and shook it. Wa-La, Rainbow sprinkles! You can pour these out on a plate to let dry for a couple minutes before mixing them together or mix together right away for more deeper colors.