Usually it is caused by rust in the system. You should have your radiator flushed regularly, there are recommendations in the owners manual for the car but your mechanic or service shop can tell you how often it should be done as well. You could also have oil in your water, this would indicate a blown head gasket.
Hi! I am a mechanic by trade and the most common reason for brown or discolored antifreeze is because someone has mixed two different types of coolant!! A li of cheverolet vehicles require a orange coolant that looks brown in a surge tank or inside the radiator!