Take the advice from those that have replied to you. I have seen both, and they are entirely correct. The lumen on the ~4000K is higher then the ~6000K HID's. And to be honest, the ~4000K HID's are already pretty blue. I personally wouldn't want them any bluer. If you ask me, the newer BMW lights suck compared to the ~4000K aftermarket HID lights in terms of output.

6000k? Thats hotter then daylight. Sunlight is usually around 5000k. I am sure you can go too far. Just as you dont want something way warmer you don't want something way warmer.

Color temperature refers only to the "whiteness" of the light, not the brightness. The higher the temperature, the whiter the light output, thus 6000K would be whiter, 4100K would be awarmer color, more yellow.

The measure for brightness ("which is brighter...?) is lumens.

I would guess the brightness of either light is greater than halogen. Can you provide the specifications from the lights you'd like to buy? Or a link to their website?
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(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)

6100K is only for 'looks'. It has a lower lumen rating than the 4100K HID lights.

The 4100K's can look a little yellowish when looking at the car, but still gve a very white light on the road. The 6100K's give a more blue/purple look, so they have the right 'image', but the drawback is lower light output.

If you do it for the extra light then go for the 4100K's. If 'looks' is what you're after then the 6100K's would be the ones.

Bye, Arno.