Like NFK said, it's more tradition than anything, and familiar lights. In the UK, we call sirens/lights the 'Blues and Twos' (blue lights, two-tone/hi-lo siren), because we're used to blue lights. Seeing a red light would probably not trigger the same response as blues. We only use reds for redirectional purposes and occasionally they'll be put on steady burn while officers patrol at night. To me, blue stands out quite well, red can blend in with street lights and brake lights but big flashy blue lights seems to work