Preheat & Prepare Pans – Preheat the oven to 350°F (175°C). Grease and line three round cake pans.
Mix the Batter – In a large bowl, mix together the cake mix, eggs, egg whites, buttermilk, oil, and sour cream until smooth.
Divide & Color – Separate the batter into three bowls. Tint each with blue, pink, or yellow food coloring, stirring until evenly combined.
Bake the Cake Layers – Pour each colored batter into a prepared cake pan. Bake for 22-25 minutes, or until a toothpick inserted in the center comes out clean. Let cool completely.
Make the Buttercream – Beat the butter until fluffy. Gradually add powdered sugar, vanilla extract, and salt. If needed, add milk 1 teaspoon at a time until the desired consistency is reached.
Assemble the Cake – Place one cake layer on a serving plate, frost with a layer of buttercream, and repeat with the remaining layers. Spread frosting over the entire cake.
Decorate & Serve – Garnish with candy eggs or sprinkles. Slice and enjoy!