Looks like this is not a bug, it’s a deliberate feature of the Nordic DLC!
Just why anyone would want grey earthworks is beyond my understanding, they are both horrible and unrealistic. The good news is that you can remove this feature by simply removing (or renaming) the ‘shaders’ folder in the DLC’s ‘res’ file.
The file can be found in your folder (Windows): C:\Program Files (x86)\Steam\SteamApps\common\Train Fever\dlcs\nordic_1\res