You'll often hear smart-alec martial arts masters say that the best self-defence technique is the 400m sprint, but they couldn't be more wrong. In truth it's the 1km parcour.
A colored flag means a sign giving a clue on something. It doesn't define a person, and can only be used to describe some abstract fact. You can say "getting angry when talking about the bear discourse" is a redflag, but saying "a man is a red flag" is not correct
Syntactically they should have used 'displays' not 'is' a green or red flag, but everyone divined the meaning of the damned sentence and the internet is garbage so who gives a rats