hmm, I don't know. State laws seems to say they MUST have a red flashing light (though maybe they do not need to use it unless they are running intersections of something?) and that police are also allowed an additional blue light:

>§ 4571. Visual and audible signals on emergency vehicles.

>(a) General rule.--Every emergency vehicle shall be equipped with one or more revolving or flashing red lights and an audible warning system.

>(b) Police, sheriff, fire and coroner or medical examiner vehicles.--

>(1) Police, sheriff, coroner, medical examiner or fire police vehicles may in addition to the requirements of subsection (a) be equipped with one or more revolving or flashing blue lights. The combination of red and blue lights may be used only on police, sheriff, coroner, medical examiner or fire police vehicles.

So my guess is they probably have a red light also but only use that one when, in addition to flagging down a specific motorist, are also trying to warn traffic around them? Blue is for you and means stop and red is for everyone else and means danger?

edit: I did find this:

>Most police vehicles primarily use red and blue lights – the red part here signifies that an immediate emergency is taking place, while blue police lights signal that police, and not other first responders, are present. In many cases, squad cars also use white lights. What do different color police lights mean?

So it seem, blue=police, red=emergency. red+blue=police emergency

edit2: I also found this:

>In Pennsylvania, the display of only blue light is reserved as a courtesy light for volunteer firefighters when responding to an emergency incident and/or fire house. There may not be any other colors present and no more than 2 flashing or revolving lights/bulbs, further it must have 360 degrees of visibility. Civilians are not required but are generally encouraged to move over when only blue light is present. https://en.wikipedia.org/wiki/Emergency_vehicle_lighting#Blue_4

note: I think "only blue" means no white flashing lights as well