I have both on my rig and I can honestly say the yellow illuminates the road better than white. Especially when they are mounted closer to the pavement. The fog light is not intended to open up visability a great distance in front of you. A correct colored yellow bulb should illuminate the reflective lines and signs on the road as well as the immediate area in front of your truck. My PIAA ion bulbs have done just that and have been worth their weight in gold on more than one occassion.
Here's a good article you should read:
http://lighting.mbz.org/tech/lights/fog_lamps/fog_lamps.html