they can be considered running lights or markers . I've seen a thousand truckers do this . Not saying it IS legal , but as I've heard in the past from being pulled over in my lifted trucks for the tires sticking out too far , markers over the offending tires can be used but must be yellow in front of the frontmost part of the rear tire and red behind the frontmost part of the rear tire . I don't see how neon lights would be any different. And I'd say pretty much that if you ask anyone who has gotten a 'neon ticket' what it was they in fact got the ticket for , most would say "display of emergency or official colored lighting" . ( I used to work it the mobile 12V industry and that was the typical response ) .

RED= emergency > Fire > Rescue > Police ( except on the rear of any vehicle )
BLUE= Police
GREEN= Forensic
PURPLE= Funeral
AMBER= public utility > general alert
WHITE= UFO .... run before they suck your brains

Usually , you'll see a mix of those ( and thats an old listing ... not sure about the green anymore ) but that was supposedly what you COULDN'T have .

There was also something about the bulbs being visible . It was the 'candlepower' argument . You cant rate the reflection of light in candlepower and the light isn't illegal if they couldn't actually see the true light it was emitting . It's an old defense and has probably been bashed to death since the last time I heard about it being used but who knows .

It is truely a grey area of the law but best bet is to not use neons unless you are precision driving a trio of black Honda Civics in an organized ring of truck theft .