They're not wearing a white uniform though... They're wearing the grey jumpsuits everyone else is. And if Snowtrooper boots are indicative of snow boots, then the AT-AT driver isn't wearing snowboots either. Just a different kind of boot... could be snow, could be boots with magnetic seals on them incase they have to leave the vehicle in a 0-G environment to work on the hull of the walker anywhere. It's just as plausible to me.
For the record, the TIE Pilot's jumpsuit (and basically every other jumpsuit the Empire has) are the same style/pattern, and the TIE Pilot's is a pressurized and heated system for deep space. So the jumpsuit could insulate you against the cold too. I think it probably does, if you have the armor/helmet/chest box thing to go with it.
I've always viewed them as just wearing gear tapped into the AT-AT's more complex systems though, and that off-duty they're rocking the black kepi and black boots like a "scanner tech", for their downtime, and Gunners are rocking the black kepi ala their look in ANH on the Destroyer, and TIE Pilots are doing the same thing, and so on. It's their version of utilities.
The red logos have always struck me as a bull**** thing too... They're Imperial Army logos supposedly, but yet nobody wears them but AT-AT Drivers? I doubt it then. To me it's more logical they're the emblems of trained AT-AT drivers or of specific Imperial divisions or something. No AT-ST driver sports them though, and nobody else seen in the movies has red logos.
The "AT-AT Drivers are in snow gear" just came about recently I think, as well... Other material said it's the uniform of the driver, required for the vehicle... The AT-AT's cockpit is clearly heated in some capacity as well, as Veers isn't wearing anything special but combat armor. Also his breath wasn't visible... My breath's visible in my car this month and we're not even out of fall.
I agree though, that the AT-AT Driver's uniform probably would heat the driver's suit, and such. It's probably ideal for cold weather, or all situations, even a planet without oxygen, or in deep space like on an asteroid's surface for instance. I see it though as a necessity to lock into the AT-AT's system... You can take it off, but operating the walker without it is pretty much impossible, ya know? And it serves the purpose of letting the pilots take a walk outside in ANY atmospheric condition, or lack of atmosphere all together.
Hell, the Wookieepedia article says that AT-AT drivers were selected from, "Hardened combat troopers"... Why?
Because people like to write things that sensationalize the mundane I guess, especially in Star Wars EU, and make things so complex it's insane.
Tank drivers aren't picked from hardened combat troops... Hardened combat troops are needed for combat leadership roles. AT-AT's are just basically tanks, with tank crews trained to run them, like real tanks get. Some are better or more experienced than others, and they're hand-picked for leadership roles in armor combat duties and such, like the tank aces of World War 2.
Now, if the AT-AT Drivers were sporting actually all white uniforms, and looked like they were actually wearing special gear that looked like cold weather stuff, I'd probably think differently of it. They really aren't though. Their boots are different though... I maintain that they could just have magnetic seals in them too though. Like I said, it's equally possible I suppose. The armor's white, but that's not foreign in the Empire either. Even "Scouts" who should be employing some level of stealth wear the white armor. Though I doubt the Imperial Army's regular divisions are so bold in the white armor, haha.