Pretty sure they will have a spectrum card to compare against. Poser lights are already illegal in just about every state, so this law is not related to that. One thing people don't realize is that a bright white light actually makes the road darker when its wet and or raining. The wet road reflects the light back so it appears darker. This is why you saw a lot of yellow "fog" lights. Incandescent and Halogen bulbs have a yellow cast to them, which is why most OEM HID are in the 4300K range. Go hotter, up to 6000K and while they light up the road better when dry, they are horrible when its wet out.
I have seen many cars with HID conversions using green, purple and dark blue lamps. I dont know how the hell they can see at night. The light output is low due to the colors. Also did you know that lights above 5000K can hurt your eyes at night when shined at you? Our eyes are really sensitive to blue, and above 5000K they tend to the blue spectrum, which is why a lot of HID produce more glare to oncoming traffic than standard Halogens.
If law enforcement ever got a really big bug up their butt, they could really nail someone who installed HID conversions. NHTSA has determined there is no such thing, and any changes made to a vehicle that change its designed functions, such as head lights and tail lights are illegal. IOW, catch a cop on a really bad day with your tail lights darkened, and he can make your day miserable. You can spend hours reading through all these regulations on cars, it will confuse the hell out of you on what you can and can't do legally to a car. Even more mind boggling is how many get away with it, cops have better things to do.
If the Feds ever wanted to have a cash cow though, imagine the hell they can rain with a department of motor vehicle enforcement where they nail everyone that has made a modification. What they consider illegal is not if something has DOT stamped on it, but it must conform to certain regulations, such as FMVSS No. 108 for lighting. Tail lights and side markers must meet a certain reflectivity standard, which includes the colors. So replacing factory tail lights which meet those standards with one that I have seen that are LED's with a chrome reflector may not meet those standards. They look cool, but if they don't meet the standards, they are illegal.