In medium bowl, beat icing sugar, 1/3 cup butter and vanilla with electric mixer on medium speed until blended. Beat in 1 tablespoon milk to make frosting smooth and spreadable. If frosting is too thick, beat in more milk, a few drops at a time. If frosting becomes too thin, beat in a small amount of icing sugar. Divide frosting among 4 small bowls, about 1/4 cup each. Add food colour to tint frosting in each bowl.