Preheat your oven according to the package directions on the white cake mix.
In a large mixing bowl, combine the white cake mix, eggs, sour cream, milk, vegetable oil, and lemon zest. Mix everything together until fully incorporated.
Beat on medium-high speed for about three minutes until the batter is smooth and slightly thick.
Divide the batter between two bowls, adding red food coloring to one bowl.
In the cupcake holders, alternately place a spoonful of red batter and a spoonful of white batter.
Use a small bowl of water to dip your fingers and spread the batter gently in the holders.
Bake according to the package directions until a toothpick comes out clean.
Beat the butter in a mixing bowl until light and fluffy.
Add powdered sugar, starting on the lowest speed, and then increase to medium.
Incorporate vanilla extract, salt, and milk or cream, beating until creamy.
Frost the cooled cupcakes and garnish with sliced strawberries and blueberries.