So the million dollar question is now: Why do BMW & Audi lights appear blue when they use a white bulb?
Well, this coloration is the result of the light projectors; the lenses: it's transparency, it's curvature, the tiny grooves etched into it; the projector assembly, the shield, and the reflector bowl. All these components work together to produce a signature of light unique to that particular optic's design. On the Audi and BMW projectors, the lens curvature at the edge bends the white light producing a "prism effect". White light is broken down to it's fundemental colors. Since blue lights is high energy, it is absorbed last and thus travels farther. So with this prism effect, you'll notice that BMW HIDs are only purple and blue from the sides, the top, and the bottom edges, but are always daylight white on the road and in the beam pattern. This phenomenon can be demonstrated when you watch an oncoming BMW hit a pot hole or speed bump in the road and the car's nose pitches up and down. The headlights will flicker and "throw colors off", but returns to a solid white beam pattern directly on the road.
Trying to emulate this color-flickering effect with a solid-state blue or purple bulb is only detrimental to lighting performance, it doesn't fool anyone, but most importantly it endangers other motorists around you. Blue light has what we call a very high diffuse density, which causes it to radiate outwards as opposed to forwards. What results is a wide glow of light outside the beam pattern that is blinding to motorists you share the road with. A blue HID bulb will produce color bleed around the headlight, around the objects it lights up, outside of the beam pattern, and around the cut off line. This is effect is known as "glare", and these illegal and improperly installed HID kits are the reason why HIDs get a bad wrap. As common evidence of glare, observe a traffic light at night in a dimly lit area. There is red light and green light. Red is opposite blue and green is next to blue, thus we can substitute green for blue. If you observe the aura, or glow, of light around a red light and compare it to that of a green light, you'll notice that the green light produces much more glare than red. Blue is even worse. Purple, the worst.
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