the most light output made for cars is had in 4100-4300k. above that, output falls off. People percieve more light from blue bulbs, but it's just because our eyes are more sensitive to blue light than any other color. There is not as much light as people think there is coming from blue halogen bulbs or high color temperature HID bulbs.
putting HID in halogen reflectors and most halogen projectors is a horrible idea, but none of you care so I'll save myself the time explaining it. Just know that it is unsafe and you aren't getting near the performance you could be getting for the same $. If you guys would wake up, you'd find out that you could get OEM HID projectors, with OEM bulbs and ballasts into your car for less than you paid for your shitty drop in kits.
If you don't believe me about the blue light thing, here's proof. 6000k on the left, 4300 on the right in identical projectors.
mainevent: yes you can use higher color temp bulbs, but look at the above pic again before doing so.
kidynomite: actually with higher color temp bulbs (8k-10k), light output is no more than weak high beams or good low beams. And with HID bulbs in halogen reflectors, a lot of that light isn't being directed at the road, so light output is no more than stock, it just looks like it is.