I'd use flat OD, but then I'm no painting expert. Early VN birds had high vis gloss OD scheme (FS 14087). This was quickly replaced with Flat Olive Drab and Lo vis markings. The nose is also flat black. Here are a few pics that might help:
This is actually one of the early YUH-1D's, but it shows the early high vis scheme:
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Here is a UH-1C of the 281st Intruders painted in the typical Olive Drab seen throughout most of the war:
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As you can imagine, though, there was some variation. Check this pic. The door is a totally different shade than the rest of the helo:
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Basiaclly, there are lots of variations on the OD scheme that can be found if yu look through VN pics. I hope that helps a little.
Ray