hello Nazgul !
(first, sorry for reply delay

)
All those models are painting with Humbrol colors. (enamel, not acrylic)
Few years ago, I had post my point of view about painting on this thread : here is what I wrote :
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- "In france in the 1/35 modelling world, there is a guy who made a little revolution in paint technics. This man is François Verlinden. He create the "Verlinden way of painting"
His paint technic made very good effects on military models : Hollows on models are fold with very liquid dark color, and surfaces and ridges are lightly drybrush with clearer color. It's not really realistic but it give beautiful effects, we can see more model's details.
Many wargamer used this technic....
The problem is that on the real military materials this is the opposit : the dust and wet mud stay in hollows materials structure while the original material colors appear on ridges !
That's I try to reproduce.
For modelers and wargamers, it's not easy to observ and accept (!) these particuliar details.
So, many wargamer prefer used the Verlinden way, because small figurines have the most visual effect.
However, for me, the good way of painting is the combination of these two different technics. It's not very easy to reproduce. I use Verliden way very ligthly with the combination of my own weathering observations. That's why, my dry brush is not very excessive."-
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I'll try to find the time to make a "step by step" personnal painting method in pictures, for one or two vietnam GHQ's model as soon as possible...
Waiting this, here are generalities painting way for my Vietnam vehicules :
1st step :
First coat of paint H116 (Humbrol Matt 116 - US dark green) on all the model.
Let dry 8 hrs or more.
2nd step :
Mix 50% thinner and 50%H33 (Humbrol Matt Black) and paint all hollows with this mixture (not painting the surfaces, only hollows).
If model is a truck, a Ford Mutt or a Cadillac cage commando, paint wheels in black with 100% Matt 33 (black) at last.
Let dry 24 hrs.
3rd step :
Painting all marks (air recognition white stars, lights, plate number, slogans,....)
Let dry 8 hrs.
4th step :
First weahering : mix 30% thinner and 70%H72 (Humbrol Matt light earth) to add dust (in fine touches) in few model's hollows where is neccessary.
If the model is an armored track vehicule, paint tracks with 100%H72.
after, (without waiting the last coat of paint dry) do a very, very light dry brush of H72 next to wheels, tracks, rear of vehicule.
Then, (without waiting the last coat of paint dry), add particular weathering, like gas dirt (in black).
All those colors coats borders lightly mix together and look like "clouds". (This is the effect seen on LVTP5's sides for example)
You can substitute H72 by H160, or H113, or H70 (red rust) on your Vietnam's vehicules, it depend of the place on the location in Vietnam (sometime, earth look like rust in many's parts of Vietnam).
5th step :
Dry brush very lightly all the top and top sides of the model with H116 (dark green)
6th step :
Paint last details, like metalic color on tracks's ridges, headlights respective color, weapons (HMG), soldiers's uniform (in M113 ACAV's turret and hull for example).
I hope this message can help you, wainting for pictures of this painting technic

Pibber.
(Long life to GHQ !)