ash gray vs greyness

ash gray and greyness both are nouns.

Word	Noun	Adjective	Verb	Adverb
ash gray	Yes	No	No	No
greyness	Yes	No	No	No

As nouns, greyness is a hypernym of ash gray; that is, greyness is a word with a broader meaning than ash gray:

Other hypernyms of ash gray include gray, grayness, grey.

ash gray (noun)	greyness (noun)
a light shade of grey	a neutral achromatic color midway between white and black