Stock lights start out at 4300k, and older bulbs tend to get bluer and dimmer over time.
4300k has the highest output (brightest) of all colors. 5000k is nearly as bright, and it rapidly drops off after that.
white light is best for visibility, the quality and rendering you get from white light is way higher than bluer light.
I personally prefer 3000k fogs and 5000k low beams for a perfect blend of colors. I think that 5000k is a bit blue by itself, and the yellow fogs help light up the road for an overall 4300k color.
Yellower light is best in rain/wet conditions, blue light is completely useless.
Hope this helps