I just completed a Spanish escopeta from TRS parts. I, too, wanted the "grey" look. Tried mustard, but wasn't pleased with the results...left patchy irregular patterns that I didn't like. Probably my fault because all I had in the house was "spicy brown" grainy mustard. And when I was through trying it, I didn't want to even look at a hotdog for two weeks. :surrender:
I steel-wooled back to clean metal, degreased, applied BC Super Blue, degreased, then applied LMF browning solution over the top of the blue. Let it work a couple of hours till it had a fine sheen of red rust. Then rubbed it back with burlap and repeated the bluing and browning over again. Finished with a good rinse in water, then glass cleaner to neutralize. Warmed the barrel and rubbed in melted beeswax sparingly, then another burlap rub. A lot easier and faster than it sounds, and ended up with a really uniform "old pewter" grey. Has just a very faint rust tinge in recesses, what I was really looking for for a gun that spent it's life in hot, humid East Texas.
If I can figure out how, I'll try to post a pic or two tomorrow