The other reason I've heard, is that people normally associate blue lights with PD. So the theory is that a unit sitting on the side of the road with 1 or more blue lights, drivers will see that and think cop and slow down move over pay attention. In most cases. Key word. Most cases.
Where yellow you see all the time in construction zones, tow trucks etc. Its so ingrained in people, that yellow and even red and white, people dont really pay attention. But a color associated with a possible ticket is a different story.