Another underused and underestimated feature of many new sourceports is the Fog/Haze feature, in ZDoom used with the Sector_SetFade ACS special. So "What's so great about having different fog/haze colors and the option to use these?" The thing about this feature is their value for the overall atmosphere of the map and even a single room.
The problem with computers is that they can't make you really feel how cold an area is or what the consistency of the air is. But with a right fog or a good haze factor, your map can achieve this to a certain degree.
Let's check shot number 1, showing the snowy icey peak from Torment & Torture 3: Just looking at it gives you an idea on the coldness of this place and its frosty and aching air.
Shot 2 shows the mining area from the same map and comparing to the first shot, this feels totally different. It's obviously warmer and beyond the air is dirty and dry, making you breath very tight.
Feel free to imagine other colors that make you feel even more different, for example dark green (like in a jungle with high air humidity), dark red (like in a metal processing installation, very warm, very hot and dirty) and even light green/yellow (bio hazard accident where the air is stinging painful).
Summary: "Fog and Haze creates temperature and consistency of the air so use it if it supports the place you are working on!"