dark red vs redness

dark red and redness both are nouns.

Word	Noun	Adjective	Verb	Adverb
dark red	Yes	No	No	No
redness	Yes	No	No	No

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

dark red: a red color that reflects little light
redness: red color or pigment; the chromatic color resembling the hue of blood

Other hypernyms of dark red include red.

dark red (noun)	redness (noun)
a red color that reflects little light	red color or pigment; the chromatic color resembling the hue of blood
	a response of body tissues to injury or irritation; characterized by pain and swelling and redness and heat