Ditto for me too.
I use Humbrol 94 Desert Tan (or brown, can never remember, it's 94 anyway) and make a green by adding black and then some white to lighten it appropriately. If I need to add a touch of blue or yellow then I do that too. The Shades varried considerably from more blue tint to more lime so you have some room to play around.
The Helmet cover has to be as Cobrahistorian described it. I've never seen any pics of modern BDU camo on a Vietnam era helmet and using it on a Vietnam figure is a pet peave of mine. I start with a very lime green base, made using a variation of the above uniform color, with a touch of blue, and paint the whole surface with that color. There are three green tones for the leaf pattern with the lighter two only differing slightly, the one being just a shade darker. I add some blue to the base to darken it and make the first leaf patterns, which are bubbly rather than splintery, reminicent of the old WWII Marine camo pattern. Then I add more blue still, just a touch though, as the middle tone only needs to varry enough to differentiate it from the light light leaf tone. Then I add
more blue still (and maybe some yellow or even a touch of black if it's getting too blue and losing it's greeness) and do the final leaf patterns. These I do sparingly as the lighter tones are the most prevalant. I've found that the brown, which is a very red brown, almost light rust or even a tannish olive drab or even orange on some examles, often only has a single splotch with maybe a tiny splinter somewhere. On my dad's old cover there is just the one, and examples I used to see at the surplus store are show a similar trend. There is also a "winter" pattern on the reverse side, using similar variances of tan-brown that are on the dull red side of the chart.
You can also use the ERDL camo Jungle Utility pattern. It looked much like the modern BDU, only a little more bubbly of a pattern and the greens were more blue, the brown being on the red side, with a lime green or tan base. The black splinters seen on todays BDUs were also present, much as they appear today. I believe the tan based version was made specifically for USMC special ops units origianlly (not sure on my source for that info, it goes back a few years), but both varriations were used by all branches present in SE Asia. I have an example of the green base version that had substantial repairs made using the tan based version. Lots of room for evperimentation there.