redness vs reddishness

redness and reddishness both are nouns.

Word	Noun	Adjective	Verb	Adverb
redness	Yes	No	No	No
reddishness	Yes	No	No	No

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

redness: red color or pigment; the chromatic color resembling the hue of blood
reddishness: The quality of being somewhat red.

redness (noun)	reddishness (noun)
red color or pigment; the chromatic color resembling the hue of blood	The quality of being somewhat red.
a response of body tissues to injury or irritation; characterized by pain and swelling and redness and heat