Why does the cooling system liquid always have a rust color?

Hello, It is not commonly known, but coolant does wear out through normal use of the vehicle, time, and depletion chemicals used to prevent corrosion. Also, if there are any leaks in the system, air will enter the system, and rust (oxidation) will begin to take hold quickly. You will need to run a complete coolant system flush, drain the block and radiator, run another, drain again, and that should take care of the issue. If that does not correct the discoloration, you should seek guidance from a professional who can inspect the vehicle. Discolored coolant will likely damage the water pump, thermostat, and lead to advanced degradation of the entire cooling system.