Redshirt comes from the Star Trek universe, it refers to a character that can be expected to die fairly early in an episode.  The term has migrated to all of SF in can even be used in common usage to intimate that something bad is going to happen to someone.

In Startrek TOS, the character’s job duties were delineated by the color of their uniform, gold for command, blue for science, and red for engineering and security.  As a plot device, or to add a sense of danger to the main characters, a red shirted member of security would often meet an untimely and early end.

Why Red?  The use of bright primary colors and be seen throughout Star Trek TOS, this can be traced to NBC’s wish to show off the then new technology of color TV.

Further reading:  Redshirt at Wikipedia, Redshirt at Memory Alpha, Redshirt at TV Tropes.