red flag vs signal flag

red flag and signal flag both are nouns.

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

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

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

red flag (noun)	signal flag (noun)
something that irritates or demands immediate action	a rectangular piece of fabric used as a signalling device
a flag that serves as a warning signal
the emblem of socialist revolution