Personally, many times I leave the metal in the white, no coloring at all and just allow these to age by themselves. Sometimes I use a cold blue/bleach/cold blue process to color the metal. I no longer brown anything.
I like the cold blue and bleach affects because each one is different, each lock, barrel, triggergaurd is different from the last one. I can get some very interesting patterns and I think it adds to the overall appearance of the project. If I don't like what I see I simply start over. I don't consider this aging, I consider this just a way to color the metal to enhance the appearence, sorta like eye candy.
As for aging, that's hard to call. That's sorta like a project call. It just may happen that a little work is needed to repair an old original. Make it all new? Mend a broken wrist, make it look new? re-cut and rebreach an old original barrel, file off that old brown finish and make it look new? Sombody hands you a bunch of parts: barrel, lock, guard all old original stuff. The barrel is old, but the rifling is still good and they ask you if you can build a gun out of them. Make it look new?
Just an opinion.......
