POLICE HELICOPTERS
UK Police Helicopters (except the London Met) have now.all changed to an all black with a yellow top paint scheme.

The idea being this is a high visibility design, working on the principle that it is easier to see a black helicopter on grey background than a mainly white version.

This is an As 355N Twin Squirrel.
There are all black civilian helicopters available which could be temporarily sprayed with a yellow top.but they are not that common.

This is an As 355 F Twin Squirrel. (The difference between a 355 N and a 355 F is that the N has bigger engines, but you would need to be a serious anaorak to tell the difference!)
The "classic" white and a stripe design whilst not technically correct perhaps, is the more recognisable to the public as they are more like Police cars.

If you are shooting at night obviously the black version just disappears.

This example has a "Night Sun" spotlight under the tail. It is a major installation and cannot be transferred to a non modified aircraft.
Again civilian 355s are available that can be dressed but all white is not common. You may have to cover over other coloured stripes.

Talk to a specialist company.

Note the As350 is almost visually identical except it only has one engine, so is not used for Police work in the UK, but makes a good double.