I did alot of research into HID systems before buying mine, and here's what I found out. ALL OEM HID systems use 4100K capsules. It doesn't matter if its BMW, Audi, Lexus...it is a 4100K system. Of all available color temps for HID systems, 4100K will put out the most light. But many people say, "I want purple lights, like a BMW". What you need to understand is that you are looking at an effect from the BMW projector lens, not the light itself. HID projectors are designed differently than halogen projectors. If you were to look at the light output on a wall, you will see that with an OEM HID system like a BMW, the light will be pure white except at the cutoff point, where there is a prism effect. At the cutoff line, the light goes from white to blue to purple. That is what you are seeing, not a purple light. The problem is that halogen projectors do not produce cutoff lines like this. Even with a HID conversion, you will not get the same effect as a BMW. For this reason, there are myriad different HID systems on the market that claim to simualte the BMW-effect. This is where you get all your 5000, 6000, 7000, 8000K, etc. HID systems from. These capsules put out light of a different color in one of two ways, either a coating of the capsule (most 5000K systems) or a change of the salts in the capsule itself. The problem is twofold. First, the light output goes down, very significantly by the time you get to 8000K systems. Second, the light is no longer white. There is a Celica in my town with an 8000K HID system. From a LONG distance, the car looks pretty cool. But when you start getting closer, the light never changes from purple, the way it does on a BMW. Then when you see the car directly from the side, you see it lighting up the road purple, and pretty dim at that, probably not as bright as the stock halogen bulbs! If you want to drive around lighting everything up some strange color, go for it. But if you're going to spend the money on an HID system, I'd say get the real thing and go with a 4100K system.
Thanks to Jesse for writing this.