I have done a fair amount of off roading in jeeps in the mountains, in the worst of weathers. Believe me, in heavy fog, if you have a white beam of light, it just adds to the fog. A yellow light beam is able to penetrate it, somewhat. For the photography enthusiasts, ever used a yellow filter to increase contrast in overcast conditions? Its the same principle.
For me, fog lights have got to be yellow. That's why I like the older benzes more.