Here in my area a car can be black in white, they just can't put any markings or equipment on it that would misrepresent what it is. A lot of old decommissioned police cars are still sold with the same paint scheme.
Speaking of old taxis and police cars, TXDot was running a drinking and driving campaign where they modified the car half like a police car and half like a taxi.
Here is the Fort Worth one:
http://www.flickr.com/photos/brylyn/3867973112/
Wouldn't want to hail that cab.