In my experience it refers to a "Reporting Person," but depending on how, where, or even in what department it is used, it could mean any number of things. There are no standardized lists of police jargon.
It's to do with the tongue.
As in talking
As in linguistics
As in bi-lingual.
Dictionary:
A characteristic language of a particular group (as among thieves); "they don't speak our lingo".
In the United States, green is only used to signify a "Command Center Vehicle" at an incident or for private security companies in some states. If you see a "Police" vehicle with green lights, it's...