I think it is a bit misleading to call the Italian case colors a "chemical process." Ruger did that, and they washed off or rusted easily. Uberti heats the parts in a bath of salts, then dunks them in cold water. Yes, this is case-hardening. It is just not bone-meal case hardening, and thus has different colors and the hardening is not very deep.
To be clear, that is what Uberti has done for many years, but in the last few years their colors have become more vibrant so I'm not sure if they have changed anything. Pietta's colors look similar so I expect they do something similar.
In the case of Miroku, the frames I have seen could easily pass for Uberti, so I very much doubt they are using bone-meal.