For true "fog" lamps to be usable in actuall fog, stay/go with AMBER.
Google why amber, but for the most effective light whic his able to cut through fog, amber works the best.
You also have a few chices as far as how to achieve the desired color.
1 is to get amber-colored lamps (some really cheap lamps have almost orange light output-beware).
2 is to get yellow bulbs (Hella yellowstar, PIA Ion cristal, etc).
3 is to get yellow/amber 3M film applied to your otherwise clear fog lamps.
In my WRX I use the combination of 2 and 3.
Yellow bulbs are never strong enough to be very effective (color-wise) but paired with film then produce a wonderfull light.
I also use 85W bulbs but not for hte reasons you might think. 55W fogs are USELESS with my HID headlights, thus going to 85W yellow fogs was the only was to make them noticable.
Now - a few reasons why you should stick to 55W unless there's aneed for highger wattage: you'll blind traffic with 85W or 100W if your lamps are crappy and aimed incorrectly; you run the risk of melting your housings and wires (sometimes leadign to ele-fires!); while you intensify the light output, the ability of cuttin through fog diminishes.
With increased output you also increase glare, and this will simply create a smoke screen once heavy fog/mist is right in fron tof you.
Hope this info help you out a bit.
In central NJ i do not run into real fog often - so my 85W fogs are rerely used. Sometimes in poorly lit areas I will use them in clear weather-but that's about it.
Also - yellows are good attention getters in heavy rain, etc. Your car/truck stand out a bit more in rain with em on.
My 4Runner has those projector "fog lamps" but the wife won't allow me to make them real yellow fog lamps.
Whta ta WAPCE she is sometimes.