Fog is one thing, and it's really crappy to ski that, as you have absolutely no orientation where to go. Flat light is the other thing. With that, good goggles help (what color of lens is personal thing so what works for me, is not necessarily going to work for you), but on the end, the only way to handle that is to learn to ski that. Best advice I ever got was from one racer ages ago. With normal light, you see track infront of you and you prepare for bumps etc. In flat light you can't so you need to learn how to ski reacting to things once you hit them. I'm sure it's easier said to be done, but that's pretty much exactly the way every racer is skiing, as most of time flat light is normal thing. You are a bit slower this way, you make more mistakes, but you just learn to react to things once you hit them. And I can tell you after a while it works. Nowadays I have absolutely zero problems riding 80 or 90km/h in complete flat light and you just deal with things once you hit them. And it works even in backcountry for me, not just on polished race courses.
But as I said, fog is different thing, at least for me, as I simply lose track where I'm going, and I normally have really good orientation feel, but in fog everything is same, so it already happened I was wondering why I'm slowing down only to realize I'm actually turning into the hill