Preheat the oven to 350°F and grease a 9x13 baking dish.

In a large mixing bowl, use an electric mixer to beat together the cake mix, water, egg whites, oil, sour cream, and vanilla extract. Mix on low speed for 30 seconds until combined, then beat on medium speed for 2 minutes, scraping edges as needed.

Pour 2 cups of batter into the prepared pan.

Divide remaining batter among three bowls. Add a few drops of blue, pink, and yellow food coloring to each, stirring well.

Place spoonfuls of each colored batter randomly over white batter. Use a knife to swirl the colors together.

Bake for 30-35 minutes or until a toothpick inserted comes out clean. Cool before frosting.

For the frosting, whip the softened butter until fluffy, then add powdered sugar and salt. Mix on low, then add vanilla and heavy cream gradually until smooth. Color as desired.

Frost the cooled cake and decorate with Easter sprinkles.