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,
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