I have Columbia Omni Heat sweat pants and a top. Much more comfortable under my coveralls or bibs/parka than jeans. I wear midweight capilene underneath.
One of the real difference makers for staying warm in a tree stand is a coverall or bib/lined overalls system. I hunted all day Friday. It was about 35 degrees with 20 mph NW winds almost the entire day...and I didn't have my Sitka fleece lined bib with me. I suffered needlessly.
Cold weather stationary or light still hunting I wear a tight fitting t-shirt and boxer briefs. Over I wear tight fitting medium weight polyblend long underwear shirt and pants. Over that on top I’ll usually wear a cotton waffle pattern long sleeve top. Finally I’ll wear camo pants and camo BDU/ACU/hunting shirt. Then either my parka/jacket and/or cold weather bibs.
It gives me lots of options on what to put on/strip off while I am hiking in and out as well as if I am going to still hunt for awhile and decide to strip off a layer or two. The bibs only get worn when I am facing like 20F weather during the day or really windy 30s weather.
I also wear 2 pairs of wool socks.
Usually I hike in with the bibs, jacket and extra pair of socks and/or ACU jacket in my pack or bundled and tied to my pack. Get to where I am going to hang a stand or ground sit, take a breather and cool down for a few and then put everything on.
That setup has been fine so far from 40s down to low 20s and a blizzard.
Colder would probably need a wool/fleece layer top and bottom and warmer boots, but probably not much else unless you were talking sub zero or very sub-zero. I’ve only worn the bibs out once so far and I was a little too warm sitting with an overnight low of 22F even after about 4hrs. Toes and nose were a little cold though. Without bibs I usually start getting a little chilly by around 9:30am or so depending on just how cold it is, high 30s and low 40s on up I generally never get cold with just the parka/jacket.
If it helps it is a Herters parka/jacket and Herters bibs. Same line of clothes for both. One of those I picked up on deep discount for like $70 for the jacket on clearance and then a couple of months later the bibs were on clearance for the same (half off each). I haven’t worn them in the rain, but the poly outter layer seems pretty water proof. I’ve hunted in a 2 inch per hour blizzard in the mid 20Fs with 30mph gusts and it didn’t soak through after 5hrs (and finally giving up). My AR-15 has about 2lbs of snow and ice caked on it (still functioned fine BTW as I took a shot at a tree stump close to when I got back to the truck because I was curious). If anything I was too warm walking around slowly (I sat for about an hour in it).
I don't think I have ever been cold with UA 4.0 base layer. Just this past Saturday I sat out there for 7 hours and other than my feet, I was very comfortable. I had a compression shirt under the base layer and just a UA jacket on the outside.