All you need is 4100K, Osram Sylvania makes them, and all the BMW, Mercedes flagship cars have 4100K
Anything higher is rice. You don't need blue, or purple lights...
http://www.intellexual.net/hid.html
"There are many companies and private merchants out there that will advertise 7000K, 8000K, and even 12000K HID kits. Most of these vendors lurk around on ebay, online car forums, websites, and ricer accessory shops. 100% of the people that buy these kits do so because they are uninformed, uneducated, or misguided in the field of lighting, and will buy these junk kits thinking three things: that these bulbs are brighter, that these bulbs should cost more money, and/or that they will perform better. All three statements are completely false. Perhaps this misconception and frenzy for purple lights originates from BMW and Audi's infamous Hella projector HIDs.
So allow me to explain the real truth of the matter... Philips is the number one manufacturer of HID bulbs. The Philips OEM D2S bulb is rated at 4100K at 12.8 volts and produces 3200 lumens of light. The Philips Ultinon D2S is 5800K at 12.8 volts and produces 2400 lumens of light. As you can see, with all other factors remaining constant, the brightness of an HID bulb declines the higher up the color index you go. Vision, a Korean bulb manufacturer, makes an 8000K bulb, which they used to advertise on Acura-Forums as 2000 lumens bright. This is barely a marked improvement over halogens, and will produce more glare and eye fatigue than it is beneficial. 4100K has been proven through tireless independent research by the Germans, Japanese, and Americans to be the most functional, truest white and thus the brightest possible color temperature (ceteris paribus).
Every car manufacturer in the world (including BMW and Audi) uses none other than a standard 4100K gas-discharge bulb. No exceptions. The reason being is that 4100K is daylight white in color and produces the same color visible light as direct sunlight. This is least fatiguing functional color on the eyes and produces the most comfortable contrast on the road."
Notice the glare, ebay kits really do suck.
Proper HID kit with no glare, very crisp sharp lines. 4100K.