A BID by police and ambulance fleets to improve residual values could make the traditional sight of blue flashing lights on white emergency vehicles a thing of the past.

Research by the RAC Foundation has recorded a trend of police cars that are silver and dark blue, while ambulances are turning yellow.

There are different reasons for these changes, according to the foundation.

A growing number of police fleets are shunning traditional white cars in favour of coloured cars in an effort to increase their residual value.

Some forces started introducing silver cars three years ago.

Cambridgeshire, Kent and Hampshire constabularies all seem to favour coloured cars. Dark blue Volvo police cars can be spotted on several motorways and silver BMWs in the Metropolitan Police area.

Motor auctions that sell used police vehicles have confirmed that silver and other coloured vehicles fetch about 10% more than white vehicles.

Police fleet managers claim they are buying coloured vehicles solely to maximise residual values.

In London, ambulance chiefs have begun purchasing a new breed of Mercedes-Benz vehicles for their accident and emergency fleet. In a break with an 80-year-old tradition, they are painted in high visibility yellow rather than white.

These Sprinter vehicles will also be fitted with time-saving satellite navigation systems and mobile data terminals.

Some claim that traditional white ambulances are being re-painted yellow to bring them into line with Europe. This new colour is being adopted across Europe to make the emergency vehicles uniformly recognisable in every country.

Research carried out by ambulance services across Europe concluded that it was important that ambulances were instantly recognisable.

Edmund King, executive director of the RAC Foundation, said: 'The days when motorists looked out for white police cars are well and truly over. They are as likely to spot a silver police car or indeed more likely to see yellow speed cameras. We do not believe there are plans to change red fire engines but stranger things have happened.'