Preheat the oven to 350 F degrees. Line a cupcake tin with muffin liners.
In a large mixing bowl, beat the butter for 2 minutes. Add the eggs, milk, and vanilla. Mix until smooth.
Slowly add cake mix and mix until just combined.
Equally divide batter into 6 separate bowls. Add a 4 to 6 drops of gel color ( a different color in each bowl) and mix each batter until color is evenly distributed. Add more gel as needed to get a vibrant color. Add the cocoa powder to the red bowl for a richer color.
Using a small spoon add a teaspoon of each color to the cupcake liners starting with purple and ending with red.
Bake at 350 degrees F for 16-20 minutes or until an inserted toothpick in the center comes out clean. Cool on rack.
Whipped Frosting
Whisk the sugar, egg whites, water, vanilla, cream of tartar and salt in a large metal bowl set over a double boiler ( a mixing pan set over a sauce pan of simmering water).
While on the double boiler, beat with a hand mixer on high for 5 to 7 minutes until stiff peaks form.
Remove the bowl from the saucepan and let cool 5 minutes. Pipe or use a small spatula spread onto the cupcakes. Decorate with sprinkles.
Notes
For a 15 ounce box of cake mix use 3 eggs, ⅓ cup of butter, and ½ cup of milk.
To make this recipe dairy-free use vegan butter and non-dairy milk.