From The Oxford English Dictionary:
blues and twos, n.
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The flashing lights and siren used on a police car or other emergency vehicle when responding to an incident.
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Etymology: < the plural of blue n. + the plural of two n., with reference to the blue flashing lights and (typically) two-tone siren used on an emergency vehicle.
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Popularized by a British television series of the same name (first broadcast 1993), which followed various emergency services responding to 999 calls.
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2010 N. Cross Captured 242 The paramedics raced him under blues and twos to the Bristol Royal Infirmary.
Link to the rest at The Oxford English Dictionary