Looking at some of the reference in marshal_banana's topic I'm tempted to say do the majority in reddish brown & use the brown to represent some random panelling or 'patched-up' areas. Would the contrast be too much to throw in a bit of dark brown too?
Also, on the photo of the brick-built Sandcrawler you posted in marshal_banana's topic, although it's very impressive, I find the rusted area a bit unconvincing. I can't quite put my finger on it, but I think it might be because it suddenly stops near the top - maybe bits of it should bleed to the top - that could be a part availability issue though. I'd also say there's too much light bley on that particular model. These things don't always look as good in a picture as they do in life though, so maybe it's bang-on and I'm just not seeing it.