Oh course, I don't know how extensively you are planning to scenic your tunnel, but may I suggest you paint the inside as well as the outside. If you don't want to fuss too much w/ the inside, perhaps, paint it a dark charcoal or brown. Better yet, if the adventure pleases you, additionally line the inside of the tunnel walls immediately inside the portals w/ stone walls or wood beam walls, such as I have pictured here from my layout, emplace a groundcover, and ballast the track some distance into the tunnel beyond the portals. In fact, a friend of mine went so far as to place tiny lighting inside the tunnels, which had a delightful effect when viewed from the outside. He felt it was well worth the effort, and that from a guy who retro-fitted the lighting!.
Frank