If you are looking for the cool HID look, the aftermarket blue bulbs are what you want. If you want a bulb that "does the job of the fog light, only better", you need to stick with yellow dichroic, or white through a yellow reflector.
The fog light bulbs are yellow because this wavelength is scattered and relected back less than other wavelengths, and penetrates deeper into the fog and rain. (yellow fog and headlight bulbs are called 'all weather' in Europe.) The blue bulbs cut out the yellow wavelengths, and DECREASE the amount of light on the road. They don't penetrate as far, and they scatter more easily. ( http://www.overboost.com/story.asp?id=102
). That is why we have a blue sky - the incoming light penetrates, but the blue wavelengths scatter more than the other wavelengths. You can have the cool blue looks, or the yellow functionality, but not both.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E