So, why is it a "red shirt"? Why red? Why a shirt? Because of this:
Red was the color of shirt worn by the nameless security personnel on the original Star Trek series. Their only job was to get eaten, shot, stabbed, disrupted, temporally-shifted, frozen, desalinated, or crushed into a cube (except for Scotty, being the Chief Engineer). Their deaths would give William Shatner and DeForest Kelley corpses to emote over, and Leonard Nimoy corpses to ... abstain from emoting over. (Gene Roddenberry inverted this trope in Star Trek: The Next Generation when he had all officers of command rank wear red shirts while the security and engineering departments wear gold — but as you can see, the theory is the same even if the colour isn't. Meanwhile, the Star Trek movies from The Wrath of Khan to The Undiscovered Country had everybody wear dark red irrespective of their position on the Sorting Algorithm of Mortality.)
