Sugoi firewall bib tights are good for 15-40F.
Any combo of knee warmer, leg warmers, and lighter weight tights for the above.
To get even colder I can put on a pair of thermal underwear under my bib tights but spending 30 min getting dressed tends to make me detour to my trainer or running.
by the time it's cold enough to cover my legs, I want another layer over my crotch too. I start with summer bib shorts, then add illuminite tights over those. Those are ok to 45+ where I'll add knee warmers underneath them. I just ordered some bellwether windfront tights because I could tell the fronts of my legs were getting colder. In your case, I'd probably start with those. I also have some sporthill XC pants but at 40 degrees I'm sweating in them.
I wear thermal tights over my cycling shorts. This has always kept me warm enough down to about 45 degrees. My feet get numb cold before my legs get cold. (Maybe this year, I'll get some warmer shoes, and I'll find out if I need warmer tights now!)