Preheat the oven to 350 degrees Fahrenheit. Line a standard muffin pan with muffin liners, set the pan aside.
Add the flour, baking powder, and salt to a medium sized bowl and whisk them together. Set them aside.
In a large bowl (the bowl of a stand mixer works well), add the granulated sugar, and butter and mix them together with the stand mixer, or a hand mixer for 1-2 minutes.
Add in the egg whites, sour cream, and vanilla extract and mix them in until they are just combined. Scrape down the bottom and sides of the bowl as needed.
Add in half the dry ingredients, and mix until mostly combined.
Add in the buttermilk and mix until combined. The batter may look curdled, but that's okay.
Add in the remaining dry ingredients, and mix until combined.
Divide the batter into 3 bowls. Leave one bowl as is, add red gel coloring to another bowl, and blue to the third. Gently fold the batter to combine and mix the color into the batter.
Add 1/2 Tablespoon at a time of each of the colors until you fill the cupcake liners about 2/3 full. Fill all 12 of the cupcake liners evenly.
Bake the cupcakes in the preheated oven for 16 to 20 minutes, or until a toothpick inserted in the center comes out clean, or with a few moist crumbs.
Let the cupcakes cool for a few minutes in the muffin pan, then transfer them to a cooling rack to cool completely.