I do only Vietnam era models. That includes figures and I have been working on the colors and the color of Vietnam mud/sand/ earth. Many different shades.
I have found that what I see in real life changes a lot when scaled down to 1/35. So the color of a shirt changes to something much lighter when it's in 1/35. I also have books and many pictures off the internet.
Once your were in country your uniforms changed drastically within a few days even. The Vietnam sun bleached everything including the tracks. Vehicles out in the field got much lighter. Something to take into consideration is what time of year it is. If it's during the rainy season everything is a bit darker not from the rain but the humidity. So shading would be different then a soldier out in the field in the sun. A uniform in the rainy season is much different. Look at pictures during the Hue battle which was during the end of the rainy season and look at stuff before. Much different.
Also take into consideration how long they were in the field.
When I was in the Army we had BDUs in camouflage and I always had a few pair for wearing in the field and for garrison which were newer and brighter. The ones for the field were washed out, same for my boots. High polish for garrison and older more broken in for the field.
So lets say you were doing a tank crew out in the field for a week or so. Their uniforms would be a very yellowish green and their boots almost gray. Everything would have red earth/mud and what was not wet would have a coating of dust. It depended on what part of Vietnam the color of the dust was. It could be a yellowish gray that turned red when wet, look at picutres of roads and that's many times what you see. On the dry side it's a yellow gray and were it's wet it's red.
What I have been trying to do is find out what unit, what year and time of year and were they where in Nam. Once you know that you will know how to color everything. A bit complex but easy enough to do.
For instance when you look at pictures from the Hue Tet battle the Marines were wearing their uniforms for a few weeks when the offensive started. So they were bleached out yet looked darker because of the rain and humidity. Many were ripped around the knees even up the legs or missing sleeves. They were very dirty from grime and sweat. Then look at some Army more south and their uniforms would be a very light green even yellowish from sun bleaching and no rain or humidity. Unless they were in the jungle and crossing a lot of the shallow ponds and water ways in the low lands.
I think the best way is to take one picture of what you want to do and work totally from that. WWII colors were completely different but could be used to mix washes depending on what you need. Dry brushing just edges with Buff color really brings out the highlights. The little buckles and things can be painted black and dry brushed with a little silver or silimar types of paint to show wear.