In early 2000s, the city of Scotland changed the street with blue lights just to improve the appearance of the city later they found that the crime and suicide rates decreases.
In 2005 Japan police set the blue street lights as well as at the Railwaly stations, afterwords crime decreases upto 9 percent. Many other areas nationwide followed that, including the addition of over 150 blue lights to an expressway near Tokyo, aimed at reducing accidents.
May be, it might be possible that the new colored lights are simply different and unfamiliar enough to cause people to feel awkward engaging in illicit or dangerous behavior where they are present.