The phrase "red and green should never be seen" is a traditional fashion maxim that originates from Britain. The phrase refers to the fashion belief that the colors red and green should not be worn together.
Blue sometimes replaces red in this advice. The phrase is sometimes modified to "red and green should never be seen without a colour in between" or "red and green should only be seen upon an Irish queen."
It is possible that the phrase may also refer to the running lights on ships, which have traditionally been red on one side and green on the other. If another vessel saw green or red lights ahead in the dark, it would mean they were on a collision course.