Sometimes cops use certian colors to mark off the "unmarked" Patrol cars.
Here in Ontario I see that the majority of unmarked cars are almost a dark purple that are on the streets. And they have a dull white/gray markings on them with the normal police markings that you cant see from farther away but you can notice them up close.
So when your looking at one flashing there lights at you up close to pull you over you can normally see that they are cops.
Dimitri