Some thoughts:
A blue bulb might make it less effective in fog, as I seen some of the best foglights in yellow ( in many european countries were they have lots of fog).Yellow being on the opposite side of the spectrum than blue, it follows that they must have that color on purpose and blue might do the opposite.
Also I wonder if HID's in your fogs wouldnt be too bright ( if you could get that set-up to work).Fogs shine more and lower on the road and while this isnt likley to blind other traffic too badly, It will reflect some in your field of vision and possibily lower your night visison adaption, especially if you run them on at nite.
Just my 2 cents.
Fred..