When a Korean’s name is written in red ink, this indicates that the person is about to die or is already dead. Red ink was used in the past to write deceased people’s names on the family register. Writing someone’s name in red ink meant they were deceased and if the person was still living you were wishing harm or death to come to them.

Source: http://creativecultureint.com/dont-write-name-using-red-ink-korea/