keep in mind, there are 2 kinds of Parkerize finishes generally available.
Manganese- usually dark black in color, but will vary to wards the Grey end on harder or more highly polished metals. I prefer this one applied over a glass bead or coarse aluminum oxide surface preparation. it gives a low gloss sheen, with an almost "velvety" look, holds oil or preservatives well. (heat to ~150 degrees and dip/soak until cool) Very scratch and holster wear resistant.
Heavy Zinc -This is the "old standby" military finish. can be light gray to bright green in color, depending on surface preparation and metal hardness.
Some contracts call for a hot caustic soda dip, to slightly etch the surface, prior to treating. this sometimes results in a slightly "soft" appearance. Usually darkens with age and oil build up. Slightly softer than manganese but "self healing" to some extent, when scratched or dinged.
Zinc is often used as a "Primer" of sorts, for a paint or dye finish, usually over a coarse media blast surface preparation. This process provides rust protection as well as a very "toothy" substrate for paint or ceramic based coatings.
Ya pays yer money, and takes yer choice...
Paul