sea green vs greenness

sea green and greenness both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea green	Yes	No	No	No
greenness	Yes	No	No	No

As nouns, greenness is a hypernym of sea green; that is, greenness is a word with a broader meaning than sea green:

sea green: the property of a moderate green color resembling the waters of the sea
greenness: green color or pigment; resembling the color of growing grass

Other hypernyms of sea green include green, viridity.

sea green (noun)	greenness (noun)
the property of a moderate green color resembling the waters of the sea	green color or pigment; resembling the color of growing grass
	the lush appearance of flourishing vegetation
	the state of not being ripe