In large bowl, combine powdered sugar, margarine, vanilla and enough milk for desired spreading or piping consistency; blend until smooth. Place 1/3 of frosting in small bowl; stir in yellow food color. Place 1/3 of frosting in separate small bowl; stir in blue food color. Leave remaining 1/3 of frosting white. Decorate cookies to resemble scarecrows. Using writing decorating tip, pipe yellow frosting to resemble hair and straw around neck, arms and feet. Use blue frosting to pipe on bib overalls. Make face, belt and buttons with white frosting.