Oh, another story in the line of "bluer is not better" - I have HIDs on my car; I ordered 4100K bulbs but they were out of stock so they originally shipped me a pair of 5000K bulbs. If you've been following all the hype, you know that these are supposed to be even closer to daylight color, and even better for visibility.
Well, oddly enough, they were true to their word, and it sucked. During twilight hours, around sunset and during sunrise, the headlight color matched the daylight color perfectly, and so you got no visual feedback that the lights were on. No visible beam pattern on the pavement or road signs. If you were unfortunate enough to be driving at these times, you couldn't see any better than if your lights were off. With a yellower headlight, (I now have 4100K bulbs, as I originally wanted) even if the light intensity is the same as before, you can see more because there's *contrast* - you can see shadows and details and you can read road signs.
Here's another fact about human eyesight - it works on edge detection. When you have sunlight and headlights both the same color, even if the sunlight is relatively dim and the headlights are relatively bright, your eyes are going to act like there's no information coming in, because there's no contrast, no edge information. When you have headlights of a contrasting color, your depth perception improves dramatically, because your brain can cross-correlate overhead shadows from the sunlight against the beam shadows of your headlights.
Granted, these periods of time are relatively short, maybe an hour or so at most, but they also tend to be intervals that you most need to be on the road, if you're commuting. It's just another factor to add to eyestrain and stress. Not good.