My 5k's give just the "right" hint of blue, IMHO, without being either obnoxious, or - degradation of good visual useful light, which as mentioned - once getting much above 4.5k, starts to make it counter productive to the REAL reasoning you should be getting HID's in the 1st place anyway, right? ...Which is, TO SEE (better)
You can tell right away they're HID's, without having to be "showy" or ricey about it. Like I say - just right. My "vote": 5k's. FWIW. YMMV. Just one of those personal preference things.
If you're buying 'em for improved visibility - going much higher than 5k is defeating the purpose (and the benefit). Buying 'em to "look cool"? - Even the 6k's then aren't even as blue as a new Audi. But that is more reflectors in the Audi than it is the bulb/light temp, which was constructed FOR the HID's. To get even as "blue" as factory stock reflector (which most are 4.x k)HID's, you have to trade off usable light to accomplish it. Not worth "chasing" if you ask me (and you did!
) Get the HID's to improve your visibility, and stay within