The British- and Irish-English colloquial phrase blues and twos denotes:
– the blue flashing lights and two-tone siren used on a police car or other emergency vehicle when responding to an incident;
– by extension, the emergency services—frequently the police.

This phrase is first recorded—in both its senses—in Police patrol: Nigel Fryatt joins the “Blues ‘N’ Twos”, published in The Autocar: A journal published in the interests of the mechanically propelled road carriage (London: Iliffe, Sons & Sturmey Ltd) of Wednesday 16^th January 1985—Nigel Fryatt had spent an evening in the company of the police in the back of Lima Two traffic patrol car around the streets of London:

Back on the street, Lima Two draws close to the roundabout south of Lambeth bridge. The peak-time lights are working and traffic is heavy. A car horn sounds, brakes squeal. “Time for Blues-and-Twos”.
Lima Two moves forward after the grey hatchback that has gone straight through the red lights.

In Duty through the night with the crews of Quebec One, published in the Pinner Observer (London, England) of Thursday 19^th November 1987, Chris Stephen—who had spent a night with the crews of Harlesden Area Car Quebec One—explicitly mentioned the allusion to the blue flashing lights and two-tone siren used on a police emergency-response vehicle:

The first warning a passenger in the back of a pursuit car gets of an emergency call is the Blues and Twos — blue lights and two tone siren.
The cruising police car is suddenly galvanised into life, you are thrown back in your seat by the force of the acceleration; seat belts tighten as the car hurtles through the narrow twisting streets.
[…]
Not every call gets the full treatment: […] although some police drivers go full tilt for every emergency, many try to grade the calls, travelling at the maximum safe speed with the blues and twos only for the most urgent. Should there be a crash, the officer must justify the speed he was travelling at.
It is an impossible equation, between the risk and the urgency of the call; an equation the police driver alone must make in a split second.

In reference to an emergency vehicle, the phrase on, or under, blues and twos means with all alarms blazing and wailing.

The following for example is from Better driving, by Emily Dillon, published in the Irish Independent (Dublin, County Dublin, Ireland) of Friday 6^th October 2006:

Always think ‘safety’ in an emergency

Deciding on what to do when you hear an emergency vehicle approaching can be a dilemma. Do you stay where you are and potentially block the progress of an emergency vehicle? Or do you move into a position that may put you or other road users at risk?
Unfortunately, some drivers overreact to emergency service vehicles travelling on ‘blues and twos’ (blue lights and two-tone horns). This is often because they don’t hear or see the emergency vehicle until it is very close, and then take drastic action to get out of the way.

And Neil Cross wrote this in Captured (London: Simon & Schuster UK Ltd, 2010),

The impact of the car shattered Jonathan’s hip and leg and wrist.
The paramedics raced him under blues and twos to the Bristol Royal Infirmary, where he lay sedated but conscious, saline and morphine drips feeding into his arm.

 

BLUES AND TWOS (TELEVISION SERIES)

 

The phrase blues and twos was popularised by the British television series of the same name, which followed various emergency services responding to 999 calls. This series, produced by Zenith North Ltd for Carlton Television, was first broadcast on the ITV network on Monday 22^nd November 1993.

The pilot episode of Blues and Twos was presented as follows:
1-: in The Guardian (London and Manchester, England) of Monday 22^nd November 1993;
2-: in The Journal (Newcastle upon Tyne, Northumberland, England) of the same day.

1-: in The Guardian:

ITV London
[…]
8.30 Blues and Twos Medevac (S)
This is Carlton’s pilot for a possible series on the emergency services and in choosing to start with a glamorous one — London’s Helicopter Emergency Service, naturals for a drama series with all that scrambling into orange flying suits — they were clearly hoping for a bit of all-action, high-speed, mercy-dash entertainment rather than chip-pan fires and cats up trees. They got it, in spades: the Bishopsgate bomb went off as they were filming.

2-: in The Journal:

Sky’s the limit for ambulancemen

London’s helicopter ambulance service is filmed while tending to the sick during its most dramatic weekend of the year.
Blues and Twos (ITV, 8.30pm) captures the excitement and chaos as the team converge on the scene of the Bishopsgate IRA bomb blast earlier this year.
Dr Karen Heath and the helicopter team also attend a serious accident involving a lorry and a pensioner, and a boy who is injured in a sporting accident.
Blues and twos is the emergency service jargon for driving with sirens and lights flashing.

In The End Column, published in the Evening Sentinel (Stoke-on-Trent, Staffordshire, England) of Thursday 3^rd November 1994, Alan Cookman gave this sarcastic appreciation of the television series:

Thursdays will not be the same now that “Blues and Twos” has left our television screens.
Until last week, this hilarious series had been a lifeline for those who bemoan the dearth of good TV comedy.
Of course it wasn’t supposed to be funny — documentaries about the work of the emergency services seldom are — but it had more laughs than any sitcom I’ve seen this year.
Tonight at 8.30 I expect millions will be missing that urgent opening music and the melodramatic voice-over insisting that “what you are about to see is real — not a reconstruction.”
They will probably recall, with tears of mirth rolling down their cheeks, their favourite moments from the series. I know I will.
Even now I can see that motorcycle paramedic speeding through the London traffic, hotly pursued by the “Blues and Twos” camera crew, desperately seeking a dramatic, life-threatening emergency, but finding only a minor accident outside a West End theatre.
An American tourist had been struck a glancing blow by a passing pedal cycle. Unfortunately, she was obviously totally unscathed, and lay in the street cracking jokes, smiling broadly and protesting that she was perfectly all right, thank you very much.
Unscathed victims do not make for good television, so the poor woman was practically nailed to the pavement until an ambulance arrived. The voice-over told us she made a complete recovery and got to see the show.
In another classic episode, the “Blues and Twos” team watched the crew of an inshore rescue craft in action as a vast armada of small boats assembled for the D-Day anniversary.
You could almost hear the sound of fingers being crossed, for surely here was a disaster waiting to happen. But if anything exciting happened the “Blues and Twos” boys must have been in the pub.
Much was made of a capsized catamaran which posed no danger to anyone, and the rescue boat was filmed racing to the aid of a windsurfer who’d fallen off his board a few yards from the shore (he made a complete recovery).
Then, at last, some real drama: an injured sailor on a large, private yacht. The inflatable rescue craft is scrambled, and the voice-over gets more and more fevered as the stricken seafarer is taken off the yacht and rushed to hospital.
Where he was said to be making a full recovery from a grazed finger.