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For blue look into the 6-8k range. Anything above that is gaudy and annoying. If you were actually concerned with being able to see at night, you'd want something close to ~4100/4300k.
IMO I would first look at what color is your car, then decide. Cause if you have a dark colored car I think 8000k will look better and a lighter colored car will look better with 6000k IMO but thats just me
From a functional standpoint, the higher you go in terms of colour temperature the more colour you obtain however the less actual useable light output you emit.
Its really a question of what you are looking for when doing this upgrade.