In a saucepan, combine rhubarb, water, sugar, lemon zest and salt; bring to a boil. Reduce heat. Cook, uncovered, over medium heat until rhubarb is soft, about 10-15 minutes. Remove from the heat and let stand for 5 minutes. Stir in strawberries, lemon juice and cinnamon. If desired, add food coloring. Cool. Serve over cake.