All aftermarket (retrofit) hid are illegal for street use, according to the DOT. That is why you do not see any of these on sale on retail stores. As chtang says, housings for hid and halogen are designed differently so that the reflection will be different. But the biggest problem is that people just put the retrofit hid on their cars without adjust them, then blind other drivers which cause accidents. That's why hid are banned over time. Do not give those "I have not got pull over by a cop yet" sh*t cause not all cops can tell the difference between normal and hid headlights. My friend had one of those color changing corner lights on his car for a year before he got pull over and fine 300ish cause it turns blue as a pattern. Just a matter of time when you do.

Back to the question. I loved my 8k hid. 5 to 6k is the whitest you can get. 3k ish is too yellow. Oh, and do not go over 8k hid, it is illegal. I read it from a post on another forum...