It was meant to represent/replicate the old plumb brown, chemical process to the metal. Pedersoli stopped doing this years ago, they now use a bake on Dura coat. I think the old stuff is better than the new paint on stuff, the finish you have can just be wiped with light oil and will continue to age, the new stuff chips and scrapes easy.
Kinda like this,
The guys shows it finished in this video, you can see the plumb tone.