Why were red and blue lights chosen to denote emergency and police vehicles?

Easily visible? And orange lights were already taken by work and tow trucks.

Flashing to get attention.
Red because is standard for stop or warning but can get lost among the other lights in heavy traffic.  
Blue is traditionally police color so they added the flashing blue to get attention. 

They use blue flashing lights on snowplows on Canadian highways.

Lots of information here: https://www.quora.com/Why-are-police-lights-red-and-or-blue 

And green for doctors and ambulances. 

I'm taking a shot in the dark here, but I'm thinking we all associate RED!!  With WARNING!  Look Out!  Our blood is red and when you see it that means danger!  Something may hurt you or some one is hurt STOP and help!  
I think BLUE because blue is a color we know and are comfortable with.  It doesn't distract from the hue of red.  I'm not sure if this reads what I am trying to express.
Blue, most of us know is the color of the water and the sky.  It is a color we can relate to and doesn't interfere with the red, warning sign.  If we put any other color like orange, yellow, or pink next to red, in the night time they wouldn't make the red stand out.

This is all said with the approach that many of us are not color blind.  I 'm not sure what the red and blue lights look like in shades of grey.  Maybe that has something to do with it, too?! This post was edited by Merlin at November 8, 2017 3:18 PM MST

Because it's somehow more visually effective than other colors. Imagine if it were black and brown. 

it might be cause people can see them better

They are opposite ends of the spectrum so they naturally clash.

So you could see them coming.