A day or two after the American public started driving, police cars became a thing. For public safety, it was (and is) important that the police be easily identifiable. There is a long history of police cars markings and design. Originally these designs were created with paint and later with high performance self-adhesive vinyl graphics.
According to "all knowing" Wikipedia "North American police cars were once noted for being painted black and white, with the car doors and roof painted white, while the trunk, hood, front fenders and rear quarter panels were painted black." Even today, police fleet vehicles typically come from the factory painted a single color, most commonly black. The contrasting white color is still added today to make the vehicle stand out from civilian vehicles. This high contrast marking created immediate recognition of patrol units from a significant distance."
Over time, police vehicle markings have become more stylistic. New vehicle colors and customized vinyl graphics the fleet graphics can be determined either by the individual agency or by uniform state legislation. In many cases, state laws establish standards for police vehicle markings. Blues, browns, reds and greens are frequently used, with brown and tans being favored by rural police and sheriff's offices.
Today, most fleet markings on patrol vehicles are created using self-adhesive vinyl. Colors are often chosen to represent the departments identity.