I find that little attention is paid to lower body backpacking clothing. What type of pants would be a appropriate for a multi day excursion with 6'' to 1' (15 to 30 cm) of snowfall and 10 to 32 °F (-12 to 0 °C) weather? I understand the basic layering scheme applied to upper body apparel, does the same apply to the lower body? Could I get away with a base layer and water resistant soft shell pants, or would a third waterproof layer be a must? Likewise, could I do a base layer with my summer rain pants, or will this option be too cold? Am I over thinking this? Should I just wear my ski pants?
Layers are useful, but I've found upper body layering more critical (and easier to alter- you don't need to take your boots off). For me, the question is how much wind will there be, and how active are you? typically for back country cross-country skiing, I'll wear light base layer, loose fitting wind & water resistant hiking pants. Once in camp & out of the wind, I might remove the hiking pants & wear either fleece "camp pants" or wool trousers. When ice fishing in the back country after getting in, I may wear the base layer AND fleece pants under the above hiking pants- it's too bulky for active work, but fine in the wind and standing around. All the above combinations have worked well in Northern Minnesota in temps ranging from +20 to 0F with wind.
It does apply, but as others have pointed out, changing pants layers isn't as easy as changing out top layers unless you have full-zip, or at least 3/4 zip pants. There are also boot zip pants, which zip up to your calf, but I'm not a huge fan of pulling my dirty or wet boots though my nice warm fleece pants.
So instead of removing pants mid layers, I typically choose my pants layers to suit the temperature and activity of the day. For alpine touring at -12°C (10°F) I would dress accordingly:
Base layer: Polyester or Marino Wool long johns.
Mid layer: Fleece pants
Outer layers: Schoeller soft shell pants and full-zip Gore-tex Paclite shells.
The only layer I change is the Gore-Tex shells; the Polyester pants are very breathable, and Schoeller is just magic, I can literally watch my perspiration freezing on top of the fabric after it breathes through. When I'm going up hill, I don't wear the Gore-Tex pants, but as soon as I get to the top, or if I get to a really windy spot, then I'll zip them on and I don't even have to take my skis off.
At warmer temperatures closer to zero, or during activities where I'm working a lot–like nordic skiing–I won't always wear the fleece pants, as my long johns generally keep me warm enough under the Schoeller pants. If I'm concerned about cold toes, then I'll always dress warmer on the bottom, like when I'm snowshoeing in my light hikers and have my toes strapped down, the extra warmth on my legs means more warm blood pumping through my feet, which amounts to cozy toes.
I personally use a base layer (which also retains heat even when wet) with the same quick-dry pants that I use for my summer hiking. I treat these pants occasionally with a water-repellant spray and this has served me well for years. I find that anything thicker, like ski pants, are overkill - just to bulky and hot.