That's the way I've always used French Gray, after case hardening.
This custom Garbi 20 ga. was pack hardened by Doug Turnbull then the colors removed with a soft mechanical abrasion technique leaving the glass hard skin. Just coloring the metal (or removing the color) without the hardness would be suspect (to me) as far as corrosion resistance(??)
Years ago I "grayed" one non-hardened action because the engraver insisted, but that action did not need hardness, and I laquered over the "gray" to prevent rust. It has held up over the years but I wouldn't do it that way again and I've encouraged the client to revisit the metal finish.
