I have a minor ( though slowly getting worse over the years) case. Started with one finger going white on occasion, now it’s more fingers, happens more often. What I have found to help is simple in concept but complex in practice, as has been described already; basically it amounts to maintaining a closer watch on my core temperature and staying on the warm side of things. While moving I have always been a furnace, and that has not change as I get older (57 at the moment) but what has changed is that the engine slows way down now when I stop, and I need more insulation while sleeping or sitting still than I used to, and I seem to cool off faster. So I have to be more proactive, adding layers as soon as I stop for a break on a cold day, or my core temperature drops and circulation to my hands drops off and next thing you know the white shows up. As long as i can stay ahead of it no problem, but I can be feeling fine, not feeling what I would describe as cold, and still have it happen. so it’s rather like trying to reset the thermostat – I am trying to get used to a warmer standard as it were. That’s the tricky part. Gloves I find make very little difference – it’s all about the warm core providing plenty of nice warm blood flow. Without that you can wear all the mittens you want and it won’t do a thing – I’ve tried it.
I had another sort of related issue on one trip where I stopped for a break, didn’t add layers right away, got chilled, started up again and had a couple fingers going. As I went up the hill I worked my hands hard on the ski poles, and did a lot of flexing of my hands, forcefully making fists and opening wide, in hopes of increasing circulation now that I was on the move again. Sort of worked, but by the next day one had was swelled up an inch all the way across the back, and very painful. It ended up going away after a day of full rest but was rather worrisome out in the wilds. On my return I asked a doctor about; he though most likely it was a ganglion cyst, and ll the unusual movement, along with the cold, had aggravated it and caused inflammation and swelling. So not exactly related, and yet the one problem had led to the other. I have since had mild issues with the cyst, always connected with having had my hands really cold. So, one more reason to be careful about it.