ok. according to the girlfriend (who ought to know)
NOT black.
Green uniforms to match the forest with soft cloth capsof the same colour.
The guerillas (not an issued uniform but normal clothes) - black 'sort of pyjamas' with a black and white check headcloth.

I was going to write the guerillas dressed to match the locals. This was a mistake. They were the locals.

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My shifting projects

NVA uniforms were a light green or green.  The VC were locals from the South, for the most part, and the "black Pajamas" were common and the head scarfs as Maddoc said and the conical "Chinaman's Hat" too ... sometimes even a "cowboy style bush hat" ...      My attempt at a Chinaman's Hat on some of my Feral Orks ...            

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Legion 4 "Cry Havoc, and let slip the Dogs of War !" ... "People sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf."

use a holepunch on some beer can aluminium? then dent using a wide pin into the right shape.

damn. so tempted but I am not building a Viet army. they're unsporting.

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My shifting projects

Yes, I only did those 3 Ork Kong ... probably will paint them in black PJs too !  But that is a good idea on how to make Chinaman's Hats ...  And they are an unsporting lot !  

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Legion 4 "Cry Havoc, and let slip the Dogs of War !" ... "People sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf."