Residential service call for a customer in The Colony and Upon arrival found that the upstairs system was not cooling properly. Checked indoor unit and found everything operational. Checked inside coil and found a decent amount of rust on coil. Checked outdoor unit and found that the Freon levels were low. Gave options on adding refrigerant, replacing rusted coil or doing a complete system change out. Homeowner opted to just add refrigerant. With system being low, adding refrigerant does have the potential to leak back out. Added 4lbs of 410-a to system to set charge. System has a supply temp of 57 and return temp of 75. System is now cooling at this time.