Rank
From Star Trek : Freedom's Wiki
In military and civilian service organizations of many intelligent cultures, rank (sometimes denoted by rank insignia on uniforms) is how the jobs of different personnel are organized in a hierarchy of authority. One is promoted on their merit, skills and talent. Colour of division is marked on the uniform, with Red being Command, Yellow being support (Engineering, Tactical, Security,Operations, etc.), Blue being Medical, Counselling and Sciences, and Purple being Civilian Services.