The green tinge seen on milsurps that were parkerized is the result of long term storage in cosmoline. M-1's, for example, were never parkerized to be green. There's an on going debate as to what colour original M-1's were. Grey or black.
However, some professionals are changing the chemicals to give a green tinge. The chemicals get changed to give black too. Regular parkerizing comes out in shades of grey. There are other metal finishes, like Luberite(a brand name for black phosphating), that are only black. In any case, the process is the same. The only difference is the chemicals used. Some of 'em do give more protection though.