Start with a tan base. Then lay in the black splotches. They should
look like an aerial view of a river delta. Then, lay in the dark green.
Bring the green splotches close to the black, but make sure to leave a
thin stripe of tan between the black and the green.
After that, lay in some light green splotches in the same manner.
When done, you should have all of the tan covered with light green,
dark green or black, except for a thin line separating the three
colors. Also, you'll want to lay in some dots of tan in the splotches.
On the bigger ones, drop a smaller green dot in the middle, so that you
have a small green dot, bordered by a tan line, in the midst of a
larger splotch of green or black.
If you're using water based paint, I'd suggest Vellejo, because it's
designed specifically for figures, and will brush on very smoothly.
I rarely use a brush smaller than No.1, but for 1/35th or 54mm
figures, you may find a 10/0 or smaller a little easier to work with.
Here's my example, though it's in 1/9th scale.
You can find some pic on the net, I'm sure, of examples of actual
colthing that will be useful. Keep in mind to that there were locally
made examples in Vietnam, so the patterns could vary in shape and
color. There was even a variation with red-brown instead of one of the
greens.