signal flag vs red flag

signal flag and red flag both are nouns.

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

As nouns, red flag is a hyponym of signal flag; that is, red flag is a word with a more specific, narrower meaning than signal flag:

Other hyponyms of signal flag include pennant, code flag, nautical signal flag.

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