redness vs purplish red

redness and purplish red both are nouns.

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