Keep it at 5000K and no higher (higher numbers = bluer tint).

I run 6500K and they are bright white with no blue tint at all. The research I did before I got them indicated that 6500K is the closest color temperature to actual bright sunlight. At night it lights up signs from quite a distance. Also the set of HIDs I got have shields around the undersides to make it so glare isn't reflected into oncoming drivers' eyes.