There were some studies done on the national level that found, if you want vehicles to move away or use extra caution, yellow or amber is best. And that's why you see the yellow lights on the back side of the most law enforcement light bars. Having red lights flashing to the rear is especially dangerous because drunk drivers will home in on the red light. Dallas had lots of cars rearended with just red lights and the frequency of that dropped dramatically when the amber lights were put on the rear of the light bars. And as to whether drivers will notice, yield, or stop better for red or blue, the conclusion was that neither was better, but to gain the most attention, either red or blue, or both, should alternate with white. So now, most of the marked cars have all three colors to the front; red, white, and blue. They'll have amber to the rear, and possibly some of the other 3 colors.