This is the lighter gray-green finishes originally used on US WWII era rifles. It provides decent rust prevention and will darken with age. The main disadvantage is that it will highlight pitting and other imperfections on a well-used firearm.
This finish became the main U.S. firearm parkerizing late in the Korean War. It is a dark grey-black and will provide superior rust prevention. Post-war M1 Garands including International Harvester and Harrington & Richardson were originally finished in manganese phosphate.
Hot Salt Bluing
This is a controlled "rusting" which provides a bluish-black finish on metal. The finish was phased out of U.S. production around the end of WWI but continued to be used on German K98's throughout WWII. The Communist Block used this finish on their AK rifles into the 1980's. It is a labor-intensive, time consuming process. This finish provides some rust protection. It is best to keep your rifle well oiled to prevent the degradation of the metal finish.