I've seen them on a single car in New Jersey once and never again in my entire life. I would consider them OK to use in crowded conditions, which is fine since they're just for other people to look at anyway, but I wouldn't use them on the highway or on fairly empty streets. Definitely hook up a switch you can toggle from inside the car.

Color is important... green is fine, just definitely do not use blue or any bluish color because that color is reserved for police cars. Yellow is fair game, but orange is for construction though it's still OK, red is for fires, blue is police / ambulance.