I don't think you need to use coolant for top-ups. Distilled water is better.

Normally the cooling system is sealed and it will only lose coolant through the overflow tank. But when going into the overflow it's only water that gets turned into steam and ends up there, the ethylene glycol (boiling point 197C) and other additives in the coolant remain in the radiator loop. Thus topping up with coolant slowly increases the coolant concentration in the engine with each top-up. Topping up with distilled water replaces only the water that was displaced and keeps the coolant concentration the same.