Red is nature's color of warning . Animals and plants with bright red colors are sometimes dangerous for other animals to eat . Many Western countries also use red color for warnings . Red lights at train and road crossings warn drivers to stop . Red , as well as yellow , road signs tell people to be careful in dangerous areas . And if you see a red flag on an ocean beach , that means the water is too dangerous for swimming . That brings us to today's expression : a red flag . If we say something is a red flag , that means we think it will likely cause problems or lead to danger . Yes , but I'm kind of surprised at her choice . Well , he doesn't have a job , for one thing . And he was dismissed from his last two jobs . That seems like a red flag . It is a red flag , if you ask me ! In this example , the speakers think the boyfriend's work experience should be a warning to Mary . We can also say something is a bad sign to mean the same thing as a red flag . Many states in the United States have laws that try to prevent dangerous people from having guns . These laws are commonly called red flag laws . When someone expresses concern or a warning over something , we say the person raises a red flag . For example , if people worry that a leader is making bad decisions , they might say those decisions are raising red flags . As you can see , we use the expression red flag in many situations . We use it mainly to warn about other people's behavior rather than natural dangers . Unless you are at the beach . In that case , a red flag means you should stay out of the water !