On Star Trek: The Original Series, if a character is wearing a red uniform, there's a good chance they be did within the hour. From Memory Alpha:
In the era regularly depicted in Star Trek: The Original Series, red uniforms were worn by members of the operations division. The duties they normally performed were in security, engineering, or other Support Services (such as communications officers, administrators, and yeomen) aboard starships and starbases.
Of these, the security personnel were quite expendable, because the Security And Tactical Division was an inherently high-risk assignment.