Q399: I have seen a car with blue neon lights under the vehicle, is it legal?
If the light can be seen from the rear of the vehicle (which more than likely will be the case with these types of lights) then an offence is committed as the only lights that can be seen from the rear of the vehicle have to be red (subject to certain exceptions, e.g. indicator lights and reverse light). If you are found guilty of this offence you could face a substantial fine.
The situation is the same for neon lights of other colours.