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

In a large mixing bowl, use an electric mixer to beat cake mix, water, egg whites, oil, sour cream, and vanilla extract. Mix on low speed for 30 seconds or until combined. Scrape the edges of the bowl. Beat on medium speed for 2 minutes more, scraping the edges occasionally.

Pour 2 cups of the batter into the prepared pan.

Divide the remaining batter between 3 medium bowls (about ½ cup each). Add a few drops of blue, pink, and yellow food coloring (one color per bowl) to the batter, stirring well after each drop, until the batter reaches the desired color.

Place spoonfuls of each colored batter in random places on top of the white batter. Run a butter knife back and forth through the batter to swirl the added color.

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

With an electric mixer, whip the butter for 1-2 minutes until it is lighter in color and a bit fluffy.

Add the powdered sugar and salt. Beat on low to medium speed for about 1 minute until well combined, occasionally scraping the sides of the bowl.

Add the vanilla and 2 tablespoons of cream and beat for one minute. (If adding food coloring to the frosting, incorporate that now.) Slowly add more cream, if necessary, to get the desired consistency. Whip for 1 more minute or until light and fluffy.

Frost and decorate the cake with Easter sprinkles.