red flag vs warning signal

red flag and warning signal both are nouns.

Word	Noun	Adjective	Verb	Adverb
red flag	Yes	No	No	No
warning signal	Yes	No	No	No

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

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

red flag (noun)	warning signal (noun)
something that irritates or demands immediate action	an automatic signal (usually a sound) warning of danger
a flag that serves as a warning signal
the emblem of socialist revolution