The colour of the light emitted is dependant on the globe, more often than not.. I would assume that the projector headlights use the same type of globe as the original housing, otherwise the wiring would have to change as well, and I'm pretty sure the kits don't come with the parts to rewire... Blue lights are illegal, depending on where you live, so make sure the lights pass the standards of your state/country before you buy them.

I may be wrong so...