Yellow fogs are used by some for style and other's under a false premise that they increase visibility. I had them laying around and did it more for style. Mine do increase visibility, but it is most likely a result of the HID's being brighter than the halogens.

Here's an interesting read on the subject:

"Why Are Fog Lights Yellow?
By David Baugher, eHow Contributor


Despite common assumptions, the yellow color of fog lights does not improve visibility in fog.

Misconceptions

The belief that yellow fog lamps contribute to superior lighting in foggy conditions is based on a misinterpretation of physical laws about the behavior of light which hold that colors at the shorter wavelength end of the spectrum disperse more easily in interactions with small particles. Longer wavelength colors like yellow were thought to be less subject to this effect.
Reality

In fact, the large size of fog droplets negates any significant difference in scattering effects among colors.
Filtering

"Yellow" fog lights are actually just filtered white lights. Since the filtering naturally reduces the emission of light, it does not boost its power to cut through mist.
Origins

The use of yellow as a color for fog lights may have started because of a genuine misunderstanding about its ability to illuminate foggy roads. After that it may have simply become traditionally accepted. An alternate theory holds that yellow is associated with danger and indicates hazardous conditions.
Conclusion

Car experts Tom and Ray Magliozzi recommend white fog lights as an alternative to yellow.


Read more: Why Are Fog Lights Yellow? | eHow.com http://www.ehow.com/facts_5899906_fog-lights-yellow_.html#ixzz1TyWur0Xn"