Blue-hued lights are completely stupid, dangerous, and (in my opinion) should be illegal.

Shorter wavelength light (blue and violet) scatters much more easily than longer wavelength light - hence the sky is blue, thanks to the blue wavelengths in sunlight getting scattered all over the place in the atmosphere. (This is also the reason blue lights won't cut through fog).

It doesn't happen just in the atmosphere; it happens in your cornea and the vitreous (clear jelly) of your eye. On top of that, the eye (like any other lens) is subject to chromatic aberration - different wavelengths focus at different angles. Long and medium wavelength light (reds-yellows) remain close together and the eye is optimized to focus there (except up close). Blue wavelengths are not focused as tightly and remain slightly blurred. In the dark, when your pupils are at their largest, this problem is maximized.

You will notice this especially when you look at a blue or purple neon light at night (maybe the "open" sign at your favorite restaurant) - it will have a major blurry haze around it.

Now replace that weak neon light with a high intensity headlight - the glare is horrendous! Even when the overall brightness is the same and the lights are aimed properly, a bluish headlight will result in much, much more glare for the oncoming cars. Some people are more affected by this than others; when I see an oncoming car with bluish headlights, the light is ALL I see. The glare/haze blocks out everything around the light to the point where I can't properly see anything else on the road in front of me.

But go ahead and use those blue lights if you really think blinding oncoming traffic is the way to go...

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