Red flag means mean, an alert, caution, or warning. When I saw him drink the brown water, I had red flags. (I was cautious, nervous, worried, that he was drinking dirty water). or, When I climbed on the roof, my mother had red flags. (She was worried about it, she had caution) I saw the red flags when the man in the movie held the girl's hand. (I was alerted, it was a clue, etc).

A red flag means an action that is unacceptable, and that warns you not to continue in a certain situation. It is an expression that has military origins. A red flag used to mean a warning that a ship was preparing to engage the enemy. en.wikipedia.org/wiki/Red_flag_(idiom) In the context of a relationship, a "red flag" means something about the other person's conduct that is unacceptable, and that ought to warn us that they are not a suitable person for us to love. If somebody ignores a red flag and does not exit from the situation, the situation often escalates and becomes worse. Example: "her boyfriend used to hit her. That should have been a red flag, but she stayed in the relationship and he murdered her". This means "her boyfriend used to hit her. That should have been a warning to her that she should finish the relationship, but she stayed in the relationship and he murdered her". Savannah, I do not think that it is correct to say "to have red flags". We do not say "I had red flags". Rather, we say "it was a red flag". In British English, at least, we use "red flag" with the verb "to be", not the verb "to have". "We can say "it is a red flag" or "it was a red flag" or "it should have been a red flag" .

Whoever wrote that text has a nasty spiteful prejudice towards people with red hair. Either that, or they are trying to be clever / funny by making a pun and saying that having "red" hair ought to be a "red" flag.