red flag vs alert

red flag and alert both are nouns.

Word	Noun	Adjective	Verb	Adverb
red flag	Yes	No	No	No
alert	Yes	Yes	Yes	No

As nouns, alert is a hypernym of red flag; that is, alert is a word with a broader meaning than red flag:

Other hypernyms of red flag include alarm, alarum, warning signal, flag, signal flag.

red flag (noun)	alert (noun)
something that irritates or demands immediate action	a warning serves to make you more alert to danger
a flag that serves as a warning signal	an automatic signal (usually a sound) warning of danger
the emblem of socialist revolution	condition of heightened watchfulness or preparation for action

red flag (adjective)	alert (adjective)
	engaged in or accustomed to close observation
	mentally perceptive and responsive
	quick and energetic

red flag (verb)	alert (verb)
	warn or arouse to a sense of danger or call to a state of preparedness