First thing I"ll say is that that's an awful lot of vehicles for a diorama. You should consider cutting it down to two vehicles to prevent "visual overload", a common mistake people make when putting together dioramas. Bigger is not necessarily better-a concise, well-composed diorama is always better than a big, sprawling scene that confuses the viewer as to where to focus.
Color-wise, just about any shade of Olive Drab would work there; preferably you don't want a shade labelled "Vietnam OD" if there is such a thing, but generally you can use something like Tamiya's OD. For WWII, I like to add a smidge of Light Gray into the OD to make it look faded, and also, vary the shade on each different vehicle for variety. Each vehicle would have different weathering and stains, etc, Don't worry too much about uniformity. The slightly different shades will add visual interest and personality to each vehicle.
Whitewash is best done with the hairspray method. Lots of info online here about that, just look it up in the Search section!