In a large bowl, stir together the flour, oats, brown sugar, and cinnamon. Using a pastry blender or two knives, cut in butter until mixture resembles fine crumbs. Press half of the mixture into bottom of a 9x9x2-inch baking pan. Sprinkle rhubarb evenly over crust; set aside.
In a small saucepan, stir together granulated sugar and cornstarch; add the water. Cook and stir until mixture is thickened and bubbly. Stir in vanilla and, if desired, red food coloring. Pour evenly over rhubarb. Sprinkle with remaining crumb mixture. Bake, uncovered, in a 350 degree F oven for 1 hour or until topping is browned and filling is bubbly. Cool about 30 minutes. Serve warm with ice cream or whipped cream, if desired. Makes 9 servings.