Due to the many varying types of duties and assignments, this is an impossible question to answer in even a general fashion. Please try to narrow down the area you are interested in.
Yes, police need to wear uniforms because you need to know who a police officer is ane their uniform should show people who a police officer is. Usually police wear a dark blue color in the U.S....
Military police officers wear military uniforms. Added: Displayed on their military uniforms are badges, similar to civilian police badges, and/or they wear armbands or, sometimes, distinctive hats...
If you are a police officer you would not need to ask. A police officer doing his duty will treat another police officer as any member of the public (if they are out of uniform and off duty).
(in the US) Blue is not universally worn as a police unifoprm color. Uniform colors for law enforcement officers vary from department to department as well as jurisdiction to jurisdiction. Many...