Why are police car lights red on the left and blue on the right side?

Why are police car lights red on the left and blue on the right side?
For the same reason airplanes have red lights on the left and blue lights on the right. If, in a poor vision situation, two police cars (or any emergency vehicle for that matter) are some distance apart and racing down a highway (or two planes in the sky) and see the flashing lights of another directly in front of them, they know by placement of the lights whether they are following the other or are in a head-on situation.