In a saucepan over medium heat, melt 3 tablespoons butter. Add brown sugar, 1/4 cup sugar and cornstarch. Gradually stir in water and rhubarb; cook and stir until thickened, 5-8 minutes. Add food coloring if desired. Pour into a greased 2-qt. baking dish and set aside. , In another bowl, combine flour, baking powder, salt and remaining sugar. Melt remaining butter; add to dry ingredients with milk. Mix well. Drop by tablespoonfuls onto rhubarb mixture. Bake at 350° for 35-40 minutes or until the fruit is bubbly and the top is golden brown. Sprinkle with cinnamon-sugar. Serve warm, with whipped cream or ice cream if desired.