In a medium sauce pan, mix sugar and cornstarch together. Add the water, vanilla, and food coloring. Heat over medium heat for about 5-6 minutes until very thick and transparent, stirring frequently. Stir in the chopped rhubarb and orange zest and then pour over the crust. Sprinkle the remaining streusel on top and bake for 30-35 minutes or until topping is golden and rhubarb is tender.