Rhubarb layer: In a large saucepan, combine the first five ingredients. Mix until smooth and then add the rhubarb. Bring to a boil, cook and stir for 2 minutes or until thickened. Remove from heat, add the food coloring and set aside.
Batter: In a bowl, combine flour and sugar; cut in butter until mixture resembles coarse crumbs. Set aside 1 c for topping. Add baking powder, baking soda, and salt to remaining crumb mixture. Combine egg, yogurt and vanilla; stir into batter until smooth. Spread into a greased 9 inch springform pan.
Topping: Combine the egg, Mascarpone cheese and sugar; spoon over batter. Top with rhubarb mixture. Add pecans and coconut to the reserved crumble mixture; sprinkle over top.
Bake at 350 degrees for 60-65 minutes or until a toothpick comes out clean.
Cool on wire rack for 20 minutes; remove sides of pan. Cool completely.