Q: I have an old side-by-side boxlock with a colour-hardened action. The action is very dirty, and I wonder how it can be cleaned?
A: That lovely brown and blue abstract pattern of colour hardening is of microscopic thickness, so you need to be very careful. abrasives such as Brasso, which are good for cleaning bright polished steel, could well do damage.
I would be tempted to use nothing more than barrel cleaning solvent. You could use a very mild abrasive cleaner such as a very fine silver polish, but strictly at your own risk.