[Orient Express]

If you plan to use your fog lamps as auxiliary lighting in rain, fog or snow then get bulbs that are 3000K or so. The yellowish color will allow you to have the best visibility in inclement weather. After all they are called "fog" lights for a reason.

But, if you plan to ride around with them on just so you can "look" like a (stylish) dumb-ass, then by all means get a white, blue or purple color.

One other comment, the only thing dumber than driving around with your fog lights on for no real reason, is to stick a HID system in a Fog light reflector. 99% of the fog lamp reflectors out there do not have an adequate reflector design for focusing the light, nor to they have shroud to cut unfocused forward glare.