Finally managed to find Tennessee's State Laws on Vehicle Lighting. It's under Title 55, chaper 9.
I started by going to MetaCrawler
and then plugged in "tennessee legislature" as a search.
That search's first result netted one entitled "Welcome to the Tennessee General Assembly" So I went to http://www.legislature.state.tn.us/
and then clicked on their TEXT MENU option.
(Note: Each state is different. For instance Kansas Legislature can be found here here
This in turn, opened a page with a search engine. After several failed attempts in hunting for "neon", I finally tried (of all things) "vehicle lighting".
That turned up a page which falls under the description: "Tennessee Code : TITLE 55 MOTOR AND OTHER VEHICLES : CHAPTER 9 EQUIPMENT - LIGHTING REGULATIONS"
Anyway, thougth I'd post how I found, what I found, as it might help others on here do the same for their states if they're curious about such. If you have no idea about what title or chapter of law relates to lighting in your state (as I didn't) I'd probably try "TITLE MOTOR VEHICLE LAW" (or a mixture thereof) and see what turns up. I'm assuming most states have some sort of similarly worded laws dealing with such.
I guess what piqued my curiosity was hearing stories where some officer has said ANY additional/accessory lighting is illegal on a car. I knew that couldn't be true, especially with all the transfer trucks running around with amber marker lights on every facet of their rigs.
Anyway, in part, here's what it had to say:
55-9-402. Lights required on motor vehicles - Exceptions - Regulations as to color, type and visibility distance.
(a) (1) Every motor vehicle other than a motorcycle, road roller, road machinery or farm tractor shall be equipped with at least two (2) and not more than four (4) headlights, with at least one (1) on each side of the front of the motor vehicle. No non-emergency vehicle shall operate or install emergency flashing light systems such as strobe, wig-wag, or other flashing lights within the headlight assembly or grill area of the vehicle.
(2) Auxiliary road lighting lamps may be used, but not more than two (2) of such lamps shall be lighted at any one (1) time in addition to the two (2) required headlights.
(3) No spotlight or auxiliary lamp shall be so aimed upon approaching another vehicle that any part of the high intensity portion of the beam therefrom is directed beyond the left side of the motor vehicle upon which the spotlight or auxiliary lamp is mounted, nor more than one hundred feet (100') ahead of such motor vehicle.
(b) (1) Every motor vehicle shall be equipped with two (2) red tail lamps and two (2) red stoplights on the rear of such vehicle, and one (1) tail lamp and one (1) stoplight shall be on each side, except that passenger cars manufactured or assembled prior to January 1, 1939, trucks manufactured or assembled prior to January 1, 1968, and motorcycles and motor-driven cycles shall have at least one (1) red tail lamp and one (1) red stoplight. No non-emergency vehicle shall operate or install emergency flashing light systems such as strobe, wig-wag, or other flashing lights in tail light lamp, stoplight area, or factory installed emergency flasher and backup light area.
(b)(3)(d) (1) No vehicle operated in this state shall be equipped with any flashing red or white light or any combination of red or white lights which displays to the front of such vehicle except school buses, a passenger motor vehicle operated by a rural mail carrier of the United States postal service while performing the duties of a rural mail carrier, authorized law enforcement vehicles only when used in combination with a flashing blue light, and emergency vehicles used in firefighting, including ambulances, emergency vehicles used in firefighting which are owned or operated by the division of forestry, firefighting vehicles, rescue vehicles, privately owned vehicles of regular or volunteer firefighters certified in § 55-9-201(c), or other emergency vehicles used in firefighting owned, operated, or subsidized by the governing body of any county or municipality.
[Ref: Acts 1931, ch. 82, § 15; 1937, ch. 245, § 5; 1941, ch. 121, § 1; C. Supp. 1950, § 2700.16 (Williams, § 2695); Acts 1957, ch. 51, § 1; 1969, ch. 40, § 1; 1970, ch. 598, § 1; 1975, ch. 342, § 1; T.C.A. (orig. ed.), § 59-905; Acts 1981, ch. 111, §§ 1, 2; 1982, ch. 850, § 3; 1989, ch. 173, §§ 2, 3; 1989, ch. 591, § 113; 1991, ch. 221, § 1; 1997, ch. 299, §§ 1-5.]
Note: What this doesn't cover is that some cities may have their own ordinance(s) against vehicle accessory lighting that doesn't show up in State Law anywhere. But at least this is a start in finding out what's legal and what's not prohibited by state law.