Preheat the oven to 350°F.

Place 12 cupcake liners into cupcake tins. Set aside.

In the bowl of a stand or hand mixer, combine the cake mix, sour cream, oil, water, egg whites, and vanilla extract. Mix on low speed for 30 seconds, then scrape the edges of the bowl. Beat on medium speed for 2 minutes longer, scraping the edges when necessary, until well combined.

Divide the batter into 3 medium bowls (about 1 cup each).

Add red food coloring to one bowl, mixing well after each drop or two, until the batter is your desired shade of red (we did about 8-9 drops total). Repeat with another bowl and the blue coloring (we did about 6 drops total). Do not put any food coloring into the third bowl of batter.

If you want cupcakes with blue on top, start by spooning 1 tablespoon of red batter into the bottom of the cupcake liners; spread evenly. Next, spoon 1 tablespoon of white batter on top of the red; spread evenly so that no red batter can be seen underneath. Lastly, spoon 1 tablespoon of blue batter; spread evenly so no white batter can be seen. The cupcake liners should be about ⅔ full. Do the opposite of this process if you want cupcakes with red on top.

Bake the cupcakes for 18-20 minutes or until an inserted toothpick comes out clean. Before adding frosting, make sure that the cupcakes are completely cooled.

Whip the butter using an electric mixer until it is lighter in color and the consistency is slightly fluffy (about 1-2 minutes).

Add the confectioner's sugar and salt. Beat for 1 minute until well combined and scrape the sides of the bowl half way through mixing.

Add the vanilla and 3 tablespoons of the cream and beat for 1 minute. Slowly add more cream, if needed, to achieve the desired consistency. Whip for another minute or so until light and fluffy.

Spread or pipe the frosting onto the cupcakes. Sprinkle tops with the sanding sugar and serve.