I would suggest generating erosion on the terrain using a dedicated tool like WorldMachine or Terragen or the like. This will make a splat map for you, which you can use in a Composite node as your layer masks. To isolate the RGB color channels you can use Color Correction nodes, or even easier just use a single OSL Uber…
http://www.moddb.com/mods/faces-of-war-the-2nd-world-war What are we looking for? - Artists with a passion for World War 2 - Dedicated members who aim to see their work finished and in game Why work with us? - We have a dedicated core team that's been working day in and day out on the project - Chance to make portfolio…