In large bowl, beat 2 1/4 cups softened butter with electric mixer on medium-high speed until smooth. Beat in 3 tablespoons milk and the vanilla. On low speed, beat in icing sugar, 1 cup at a time until smooth and creamy. If frosting is too stiff to spread, add additional milk, 1 teaspoon at a time. In medium bowl, place 3 cups frosting; tint with pink food colour to desired colour. Divide remaining frosting between 2 small bowls. Tint 1 bowl blue and 1 bowl yellow by stirring in food colour to desired colour.