Meanwhile, in large bowl, beat butter with an electric mixer on low speed until creamy. Beat in powdered sugar, 1 cup at a time, until smooth. Add 1 tablespoon vanilla and milk, 1 to 2 tablespoons at a time until well combined. If frosting is too thick, beat in additional milk, 1 teaspoon at a time. Reserve 1 1/2 cups of the frosting in small bowl. Add blue food color to remaining frosting; stir until blended. Add 1/2 teaspoon yellow food color to reserved frosting; stir until blended.