redness vs dark red

redness and dark red both are nouns.

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

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

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

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