Originally Posted by Henry H
Yellow lights or yellow film on fog lights have no proven benefit while driving. It does not increase visibility nor "penetrate fog." I don't know when the marketing for yellow fogs came about but any benefits is little to none. Fog lights are just nice and low to illuminate as much of the road while limiting amount of light reflected off the fog (glare).
It doesn't penetrate fog better, necessarily, but it does make it easier for people to see in poor weather. It has more to do with the way our eyes see light.