From what i've been informed on a few occasions by police when asking about the matter.
Neons are okay under these conditions:
1) They are not blue/red/white outside the car. This is because they are the same colours as emergency services.
2) They can't pulse. Similar to above.
3) The tube/light itself can't be visible outside the car. EG: You can have a neon under the car, but can't be so close to the edge that its viewable. You should only be able to see the light from it, not the actual tube itself.
4) It can't be distracting to other motorists. Now from what I can gather, this is up to police discretion. You can have neon's, but obviously you cant have so many that your whole car glows like a las vegas strip club. When talking to the police they have informed one or two under the car or a couple in the drivers and passenger footwell is fine. Just don't try make your car a christmas tree.
This has been from my experience from talking to a few cops. Of course when not driving you can have as many on as you like, but while driving its a lot more restricted.