The "hero costume" was a tight spandex undershirt, with a cotton/spandex mock-turtleneck "t-shirt" which I believe fastened to the standard-issue wool-gabardine jumpsuit at the chest.I noticed they had several different varieties of the violet-grey shirt worn underneath. A short sleeve version all the way to the elbows for. Chakotay, a tailored, shorter sleeved version for Tom, a short sleeve version for Janeway, and a tailored tank top for B'Elanna.
When worn with an open "jacket", the turtleneck fastened to the trousers so as to keep both the shirt and the pants looking neat and crisp.
I don't recall if the turtleneck was seen "untucked" at all.
The undershirt was seen as a crew-neck tee or sleeveless depending on the actor (probably a male/female thing) in extremely disheveled situations, for example in Year of Hell Janeways uniform jacket got ruined by Neelix's Vogon Stew so she just went around in a tank top.