For pudding, in a heavy medium saucepan stir together the sugar and flour. Stir in the skim milk all at once. Cook and stir over medium heat until mixture is thickened and bubbly. Cook and stir for 1 minute more. Remove from heat. Stir in margarine, vanilla, and, if desired, food coloring. Spoon pudding into a bowl; cover the surface with plastic wrap. Chill in the refrigerator until serving time.
To serve, peel and slice bananas; reserve 4 slices for garnish. In 4 parfait glasses, layer half of the pudding, the crushed wafers, and the banana slices. Top with remaining pudding. If desired, using a sharp knife, carefully cut the chocolate wafers into quarters. Tuck one piece of chocolate wafer into each serving. Garnish with reserved banana slices. Serve immediately. Makes 4 servings.