Well, I checked out the Law in California & here is what it says: (Read the last line)
Headlights California Vehicle Code Sections 25950 & 24011 & California Code of Regulations Title 13 Section 692
All light projected from a vehicle to the front of the vehicle may be either white or yellow. Even though it is common to see vehicles equipped with purple and blue headlights, these vehicles are probably in violation of this section. Look for the DOT symbol on the bulb and package. Headlight covers are also not legal.