They would not have "stained" (as in for appearance, per furniture or millwork) wooden coaling towers. I would imagine that creosoting played a role in their construction. Never trust the color of a plastic kit. The tower wood would (wood would?) age, of course, and brown, black and gray would all play a role in their colors. The black would come from general environmental grunge, plus the smoke from the steam locos.
One of my favorite steam-era colors for rail, ties, creosote structures and the like is from the flat camouflage line by Rust-O-Leum or Krylon, called "Camo Brown" or just "Brown", as the basic color. It's what I call a "black brown", but that's just me. Some black and gray dusting adds to the "used industrial wood" effect.
I imagine that there were variations.