Layering with multiple items is better than wearing just one or two heavy layers. Synthetics are the way to go for comfort, moisture wicking and warmth. In August, bring lighter weight material; in mid-September and later, you should bring heavier weight underwear.
Try products made with Thermax, MTP, Thermastat, Capalene, Polarmax and Micro Fleece. Polypropylene is not as good as it holds odor and should not be dried in a dryer. Merino Wool underwear can work well; Smart wool, Ibex Wear, Arcteryx RHO wool and Ice Breakers are great, especially in late September and October.
I prefer neutral colors: gray, brown, green or black as they can be worn by themselves. If you have long legs, get tall bottoms or you will experience high-water syndrome. Tops with a zip-top and longer neck are warmer and more comfortable when wearing binos. Arcteryx, Cabela’s & Patagonia made great synthetic products aside from the great ones Sitka makes.
Regarding underwear, briefs or boxer briefs will work. I like boxer briefs when worn alone with just pants or regular briefs when wearing long underwear bottoms. Make sure that whatever you wear does not chafe your privates (some guides call this “Baboon Butt”) as you will be walking bow-legged and your smile will fade away (this is why I recommend a tube of Tinactin in your daypack). Some people don’t wear underwear underneath long underwear bottoms. Cabela’s MTP, Apalene, Polarmax and Sporthill make great synthetics. Ibex Wear also makes a set of Wool,no-itch briefs worth trying.