Purdey uses Johnsons. As does the vast majority of English makers making high quality guns. Rust bluing or blacking on British guns barrels was rarely if never a "satin, dusky or dull" finish. 99% of the time it was highly polished, shiny blacking. For some reason, Americans got it into their heads that the proper finish for rust blued barrels was that satin, dull look. Not proper or correct.
Colt "Royal" blue is furnace bluing aka Carbonia Blue. The peacock/sapphire/translucent blue is nitre bluing (which is done with salts) or on smaller parts "fire bluing" which is done with a torch.
