Depends on the airfield. Last time I was over there (Summer 2001) the concrete was almost white. One thing to remember about an OIF / OEF diorama (and I've been to the Persian Gulf 5 times) is that the sun bleaches EVERYTHING!!! Most of the tarmac areas are newer, especially for OIF stuff, but I wouldn't go straight black, maybe Gunship or Euro I Gray. The concrete slabs I would use maybe Armor Sand and then highlight with Camo Gray (something a little less stark than Flat White).
I'll see if I can dig up some photos for you from my "pre-digital" camera days. I do know from being on the USN ships out there that sand blows everywhere and has the effect of sandblasting on handrails, etc... so that might be something to think of when weathering the ol' Warthog... Cheers!
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