Richpierce..bingo...more honest words were never spoken...it's an art. Many of the components and materials of ageing procedures of the best rifle builders in the business come from art supply stores. Dyes, pigments, tints, dryers, applicators...alcohol based, water based, modern acrylics and back yard and farm debris can be found in most shops. For some reason, most of these artists have burnt umber, carbon black (mine comes from my own flue and I use a mortar and pestile), store-bought and naturally-derived pigments, acids and bases. They take advantage of heat and time, and experiment with their own approaches.
I have a feline in my workshop and we formed a partnership for this aspect of the business. To begin the process of dirtying up the metal, I drop a few small pieces in the litter box and she takes it from there. They stay there until I change the litter; I didn't say this was fast. Trust me, you won't get a uniform appearance. You will then get a different result from browning solutions used later on. I don't profess to know the chemistry here, but the results seem to be potentiated if the cat has her ration of beer foam. I have succeeded in convincing my wife that this is important to the overall process.
I asked one of the finest builders which brand of black spray-paint he uses as the base of that fine aged finish where 90% or so is backed off to leave a finish darn close to an original. This, of-course, being done on a stock that he had already been finishing for many days. His answer, "whatever is on sale at Home Depot." So there's a trade secret from a best-grade contemporary gunsmith.
I do not profess to know what all the outstanding gun builders do in their finishing procedures, but, I do know this: some are masters and their artistry can not only fool the novice, but it can also baffle the experts. It's that good. Any collector who tells you he hasn't been fooled, at least temporarily, has either not seen many good guns or else likely suffers from selective memory. But don't feel bad, I know paid professionals who have changed their opinions of an item once they literally walked outside and saw it in the sunlight.