Try adding a little white to your olive drab. Now spray the surfaces most exposed to the sun. Keep the lighter color toward the center of any panels, and leave the panel lines a little darker to allow for some contrast. You may want to lighten again after drying, depending on how faded you want the paint to look. Put on a burnt umber or black wash in all the crevices. Then start your dry brushing with an even more lightened olive drab. You might try going over it a second time with an armor sand, or one of my favorites, radome tan. Throw in some silver highlights where paint would have rubbed off. Do the same for the wheels and tires. Depending on whether the vehicle was stationed will depend on the color of chalks (pastels) you would add. For example, in parts of Southeast Asia (Vietnam), the dirt was red, so you would want to put some reddish pastels around the wheels, tires, and lower portions of the body where accumulations of the stuff would occur. And during the rainy season, it all turned to mud. There's a lot of room for artistic license here, so just have a good time with it....and post pics when you're done!
Hope this helps.
Gip Winecoff