4000-4300 is oem spec and is bright with a hint of yellow but only in comparison to another colour kelvin of another hid bulb
6000k is more white with no yellow and is barley less bright than 4000- 5000k
8000k is ok for look and ok for driving but nothing special over halogen
10000 and 12000k are for look nothign else ,worse than driving with halogens
i had a pair of 8000k and they worked ok but i hated them since i had 6000k before which would let me see all of the road.