Every car manufacturer in the world (including BMW and Audi) uses none other than a standard 4100K gas-discharge bulb. No exceptions. The reason being is that 4100K is daylight white in color and produces the same color visible light as direct sunlight. This is least fatiguing functional color on the eyes and produces the most comfortable contrast on the road.

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.