The Mountain Hut
The head is the most important thing to keep warm when your feeling a bit chilly. Think of the head as the HEAT radiator(like the hot plates on the backside of an air-conditioner). You can simply put on a hat to warm up(assuming everything else is taken care of). Then when your feeling warm, take off the hat. Theoretically it should be PLENTY cold enough to freeze you up in a few seconds to a few minutes when you remove the hat, otherwise you have too much 'other' gear on. try stripping some insulators on the top and bottom if you find that the hat trick isn't working as your thermostat. Once you have things adjusted, first its the hat that comes off to cool off while hiking, then if this isn't enough, move to the armpit zippers immediately, then to the chest zippers and make sure the parka is unzipped in front all the way, then move to the legs and start to unzip the side zips to vent the legs. If this doesn't cool you off, then be sure to use only light-weight thermals on top and bottom when you hike instead of the warmer mid-weight or expedition-weight ones. also go to mid-weight socks if you have expeditions on. Once you thin out you're clothing and fine tune your venting, the hat should be all you need to adjust when hiking, when your not adjusting your vent zippers. This makes for good perspiration control ensuring ultimately that you stay warm the whole trip, and that you don't have to hike any frozen water logged clothing out. A frozen normal 7lb sub-zero goose-down sleeping bag can become over 20lbs and impossible to thaw in arctic conditions during an extended stay due to frozen locked perspiration, so I cant reiterate enough how important it is to regulate your heat, which is ultimately regulating your moisture shed. The HEAD insulator is just the easiest tool to slip on and off to help regulate this.
I personally use a wool balaclava for the insulation layer. It has its upsides. You can breathe through it comfortably all day long, and the material doesn't have the best elasticity, which causes the mouth piece to sag perfectly where I can breath thru it(comfortably) only if I need to, and it doesn't hang too low onto my eyebrows to bother. But I can think of some more comfortable ones, like pure soft fleece synthetic types. There's also those neoprene wind/face shields(some are camouflage) that work great too for frostbite conditions as well. And of course, this balaclava-style-hood insulator-layer is actually the thermal layer once you throw a down or synthetic insulator hood over that for more warmth( I have both a down, and synth, stand-alone hood that I like to bring with), then your parkas shell hood goes over that in storms. A beanie is ok to bring with, only if you don't have a good balaclava, but FULL face protection is a must, and a beanie/parka combination offers little face protection without the balaclava or other type of full face mask(synthetic, neoprene type, or wool/wool-synth-mix).