I have seen some information on the LDRG Chevy trucks. I liked the pink and olive paint scheme. What would the color of the undercarriage,axles,transmission and engine have been?

Same as the vehicle colour:

Dare I suggest 'dirty'.
The logical answer is that it depends on the colour of the originally supplied Chevrolet. If US OD then I'd suggest that inside the engine bay and under the bonnet (hood) are likely to have remained OD green. The engine itself is unlikely to have been repainted.
As to the chassis and running gear....quite probable to have been repainted by base workshops prior to issue to LRDG or a unit from whom LRDG acquire their kit.
Check out www.lrdg.org

Thanks. With Armor just paint the underside and inside the road wheels the base color and weather it. However the underside of trucks are filled with breaks, exhaust, cable, wire springs transmissions, oil pans and an assortment of other small parts. I don't know if the factory just painted everything one color or did the underside not matter and only the topside was painted OD or what ever the base coat.

The Chevy 30 CWT was a specifically made for Britain version of the civilian vehicle. Being made in Commonwealth Canada, it would have been painted in colours as per British theatre requirements pre delivery like all Canada delivered vehicles- i.e. not US OD. Never the less, the LRDG removed and modified, or replaces, major components on the vehicles upon receipt, and they would have been painted then.

The axles and differential housings, springs, etc, being exposed to view were painted as per the pictures. Engines, gear boxes, etc would have remained original colours, being mostly hidden from view, especially on the 30 CWT.

Exhaust pipes, etc would not be repainted, however any cables would be overpainted if located where other components would have been repainted.

Here is a Chevy 30 CWT front section found many years later ( the '70's I believe) showing the inside of the headlights and behind the grill. This area was clearly painted as per the paint remnant seen.
http://www.ww2incolor.com/modern/lrdg_nz_truckpresent.html

and this showing the firewall as painted, but chassis and engine mount area in the original black
http://imagerover.photoshelter.com/image/I0000VoXwIkAUNHQ

There are wartime photos of other LRDG vehicles showing firewalls painted when the engine covers were removed...

Were all or some or just this one have a black firewall and chassis?

Civilian trucks of the era had black painted chassis and engine areas. Often the interior was black, as were the fenders. The truck in the picture could show the original paint with the LRDG sand weathered off. It looks oddly shiny.