In a small nonreactive saucepan, combine the ketchup, vinegar, sugar, soy sauce and 1/3 cup of cold water. Bring to a boil over moderate heat, stirring. Add the carrot, green pepper and scallions and bring to a boil. Stir the cornstarch mixture, then add it to the pan; stir until the sauce boils and thickens, about 30 seconds. Remove from the heat and cover.