redness vs purplish-red

redness and purplish-red both are nouns.

Word	Noun	Adjective	Verb	Adverb
redness	Yes	No	No	No
purplish-red	Yes	Yes	No	No

As nouns, purplish-red is a hyponym of redness; that is, purplish-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
purplish-red: a red with a tinge of purple

redness (noun)	purplish-red (noun)
red color or pigment; the chromatic color resembling the hue of blood	a red with a tinge of purple
a response of body tissues to injury or irritation; characterized by pain and swelling and redness and heat

redness (adjective)	purplish-red (adjective)
	of red tinged with purple