4
Meanwhile, in medium bowl, beat powdered sugar and butter with spoon or with electric mixer on low speed until well blended. Beat in vanilla and 2 tablespoons of the milk. Gradually beat in just enough of the remaining milk to make frosting smooth and spreadable. Divide frosting among 4 small bowls. Stir 6 drops yellow food color into frosting in one bowl. Stir 4 drops red food color into frosting in second bowl. Stir 6 to 8 drops blue food color into frosting in third bowl. Stir 4 drops yellow and 2 drops red food color into frosting in fourth bowl.