From pictures of old anthracite mines, it looks like the outside of some are made of vertically mounted wood planks. Don't know if they were painted, but If not, i assume the wood is the grayish color of old barns (diluted indian ink?). They would probably be well maintained by a large coal company (Reading Company) if so, any idea what a typically color might have been?
http://freepages.genealogy.rootsweb.ancestry.com/~nepaphotos/november/November1/Bear%20Ridge%20Colliery%201907.jpg
greg - Philadelphia & Reading / Reading
Most wood that was exposed to the elements was treated with creasote as a preservative. (Believe the EPA frowns on this stuff today.) When fresh it is very dark brown, nearly black. As it aged it lightened in color to a gray, often streaked where it had soaked in. Common use was for telephone poles. I don;t know of many applications where it was reapplied, the timbers, posts, whatever were replaced when they finally rotted out.
Have fun,