“Parkerizing” or Phosphating is different process in which either zinc or manganese phosphate is used to add a protective coating on metal parts. The temperature the solution is heated to is much lower than that of the Caustic Salt Bluing process and it is an acid process where hot salt bluing is caustic. Parkerizing has been around since the 1800s and is most common on military firearms. But is also popular on firearms that a greater level of protection is required. The preparation can be fast as the phosphates will not adhere well to polished parts so parts are generally just bead blasted. The green/grey color seen on many older firearms is not the actual color of the Parkerizing as all Parkerizing or phosphating starts out black. The color you see is as a result of the oils and cosmoline left on the firearms over years of storage.
Very rough High Power that I parkerized and blued.