Overall RSOV (Ranger Special Operation Vehicle) color?

They are painted flat black, which quickly fades to a dark grey.  Model Master Gunship Grey is a good starting point.

Polly Scale Grimy black would be a good alternative.....I try to keep away from enamels

They are indeed black, do not know why Hobby Boss specifies dark grey.  While I am glad Hobby Boss decided to release the Land Rover series they really could have done a much better job in getting the details on the different variants correct, including colour schemes.  Many times I think Hobby Boss relied on prototype and company demonstrator  vehicles for information instead of actual in-service vehicles, especially in the case of the RSOV.  It may appear to be a dark gray after fading, but most of the photos I've seen still show them to be black.  I plan to paint mine flat black using White Ensigns British Blue/Black Disruptive to try to simulate the strange coloration of the black (the British have the same 'problem' with the black they use with  the green/black camo scheme they have used for decades), but to each his own!  Hope yours turns out well!!  

I decided to go Model Master gunship gray to give it sort of a dark blue gray faded out look. Maybe I'll use some Polly S grimy black for contrast .  Some how doubt these vehicles are repainted very often.