Drain pineapple, reserving juice. Set pineapple aside. Add enough water to juice if needed to measure 1 cup. In a large skillet over medium heat, cook the meatballs, green pepper and juice mixture until heated through., In a small bowl, combine the cornstarch, cold water, vinegar and soy sauce until smooth. Add brown sugar and reserved pineapple to the pan; stir in cornstarch mixture. Bring to a boil; cook and stir until thickened, about 2 minutes. If desired, serve with rice and top with green onions.