Hi Tingler, and welcome to the group!
As .60 said, the finish depends on the intended mission of the aircraft. In the early days of Vietnam, most helicopters in Army inventory wore a High Visibility (HiVis) glossy finish, but once they realized the sparkling reflections wasn't conducive to coming back in one piece, the Flat, non reflective markings were used. It wasn't an instant change over, however, and sometimes aircraft had a mixture of High & LowVis markings, especially from 1965 to 1967. Generally after late '67/early 68', all Army helicopters in combat were LowVis.
Back then, Bell & Boeing & Hughes & Sikorsky, always liked to show off their new aircraft by sending them on "tours" to different Bases around the Country for photo ops and public affairs purposes; these trips were normally made by factory fresh aircraft in glistening Emeron. (see below)
Emeron paint is rather expensive, however, and doesn't make a good base coat for the OD application... (after a few times wiping off spilled hydraulic fluid, the HiVis markings start coming through again (see pic below), so instead of waisting all that time & money painting them gloss, they were given a dull coat instead.
Helicopter Units stateside followed suet, with the exception of Training Units, which sometimes combined the big Day Glo orange panels along with the gloss & flat applications. (see below)
I remember one of the "D" Models we got ferried around in the last time I was at Rucker, was flat, light OD overall, with each of the sliding doors painted Day Glo My advice, is to locate a picture of the aircraft you're working on, for the time period you're wanting to capture, and build it after that photo. Can't go wrong that way.
I don't think I ever saw a glossy UH-1C however.... there may have been, but if so, they were sure elusive!
Take care and good luck,
Frank