In a saucepan over medium heat, combine the cranberry sauce, sugar, vinegar, ginger, mustard, cinnamon, and pepper. Stir over medium heat until smooth. In a small dish, stir together the flour and cold water. Stir into the cranberry sauce. Bring to a boil; cook and stir for 2 minutes. Transfer to a serving bowl and stir in red food coloring if using.