My guess is that if the German's can take Amien's, they would be in a very good position to negotiate a peace treaty. They aren't likely to get much, but potentially something along the lines of: Brest-Litvosk recognised in the East; no colonies returned to Germany (because they have no hope of reclaiming them); no reparations or war guilt acknowledgement by either side.
The American's would be most willing to negotiate, with Britain being the second most likely, and France highly unlikely being willing to negotiate unless forced to by the other members of the Entente
Why wouldn't they be able to regain their colonies? Granted, South-West Africa and its Pacific colonies are a completely moot point, but German East Africa hadn't surrendered, and Lettow-Votbeck's forces were still fighting on, so I don't see why they wouldn't get German East Africa back at least.
Also, would they be able to coerce the Congo from Belgium?