TOM: So the question becomes, to yellow or not to yellow? There's a lot of debate about this, but the research says that yellow lights are no better than white lights at penetrating fog. The theory bandied about was that yellow light has a longer wavelength and is therefore less likely to be reflected by the fog particles. Turns out, this is complete poppycock.
RAY: Apparently, the fog particles themselves are so big that they reflect all colors of light. Basically, all light bounces off of them, so using yellow light instead of white light gives you no advantage.
TOM: Plus, in order to get yellow light, what fog-light manufacturers do is put a yellow lens over a white light. That cuts your light output by 20 percent to 30 percent, which is counterproductive.
http://www.berkeleydailyplanet.com/article.cfm?archiveDate=02-15-02&storyID=10186