Your basic bunker piece code:
Code:
AddTerrainObject = TerrainObject
   PresetName = Closed Tomb
   AddToGroup = Bunker Modules
   GoldCost = 50
   FGColorFile = ContentFile
      Path = Base.rte/Scenes/Objects/BunkerModules/TombAFG.bmp
   MaterialFile = ContentFile
      Path = Base.rte/Scenes/Objects/BunkerModules/TombAMat.bmp
   BGColorFile = ContentFile
      Path = Base.rte/Scenes/Objects/BunkerModules/TombABG.bmp
The first FilePath directs the bunker piece to the BMP that overlays colors over the bunker material. The second FilePath directs to the BMP that is composed of the materials that make up the bunker, based on the RGB values in Base.rte/Materials.ini. Finally, the last FilePath is what directs what BMP to use for the background.
You can see examples in Base.rte\Scenes\Objects\BunkerModules.