I would match the playfield colours under the posts, Blue for the blue water areas, white for white water areas, orange where there are rocks on playfield under the posts.
Unless the rubber on the posts is starting to crack/flake off. The black track are actually carbon from the contacts in the machine sticking to the ball by static. It is in the ball trough, under the plastics and probably stuck inside all the light inserts under the playfield. If you tear the playfield down and clean it up it will never get as bad again.
I think that would look pretty sweet with custom posts.
Great looking pin whatever you decide.