I have had some experience with blueing and I know that preparation is really important. I know that controlling the conditions for either cold or hot is also something to be really careful with. I also know that there is more than one process you can go through to achieve results.
I wanted to learn a bit more so I started doing some reading on the net about parkerizing, browning, blackening and was learning more about the chemicals, temperatures and what-not when I ran across two articles that made me wonder...
Brown, black, or blue...or what about some other color?
Now I know one can anodize aluminum into all sorts of fancy colors, but is there a way other than painting to get similar results with steel? Not that I have any idea of how to anodize...that might be next.
The first article mentions green.
If the blueing solution is old, greenish colors may result.
The second article mentions purple.
If the "blue" is purple toss it back in and raise the temperature 5 degF
I think that both of these would be interesting for a barrel. Both discussions mention these aspects as imperfections or mistakes, but would there be a way to make them consistent enough for finishing and protection?
I did find one other website that does a "greening," but I am hoping for some way to do this myself. http://www.bearcoat.com/prodserv.htm
It almost looks like paint.