In general, how are the chassis of military wheeled vehicles (hummers, trucks, etc) painted, colored and weathered?

From the factory the chasis is normally the same color as the base color of the vehicle. I am not sure about the newer vehicles of today .

For HMMWVs, the frame is black, the underside of the truck is the same base color as the truck's topside. If the truck was manufactured in NATO colors, the underside is green, if the truck was manufactured in desert sand, the underside is sand as well. If the NATO colors were overpainted sand, the underside was not painted and remained green with a black frame.

For other wheeled vehicles, the underside is forest green.

I've noticed at museum collections that the same applies to the Soviet BTR series of trucks too - black frames and wheel hubs - in many cases. I've seen a Czech SCUD-B with a black frame as well.

Gunnie