Blues and twos create an interesting atmosphere at night.
Although British, and arguably informal, blues and twos refers to the lights and sirens together.
blues and twos
1 The flashing lights and siren used on a police car or other emergency vehicle when responding to an incident. Also later "on (also under) blues and twos".
2 The emergency services (frequently the police).
1980s; earliest use found in Autocar. From the plural of blue + the plural of two, with reference to the blue flashing lights and (typically) two-tone siren used on an emergency vehicle.