The phrase "red shirt" typically refers to a character in a story or show who is likely to die. It became popular because in some TV shows, characters wearing red shirts would often be killed off to generate suspense and drama. Full definition
It takes a ton of inner strength to watch 4,000 + people in red shirts outside of your window protesting while a very heavy police presence looked on. (educationnext.org)
Students described shooter as wearing red shirt and black vest. (ibtimes.com)
Love how you mixed jeans skirt with red shirt, looks super stylish on you! (thekontemporary.com)