Anything over 6000k requires coated (painted) bulbs to achieve that color. 4100-4500k will give you the best visibility because a) the bulbs are not coated and b) it's closest to the color of sunlight, which the human eye has adapted to over many millions of years. 6000k is something like 30% less effective brightness vs 4100-4500k. Anything coated is drastically less output, often less than stock halogen when the color goes up past 10k. It may "look" brighter when you stare at the light, but with regards to what it actually lights up on the road it will be less effective.