sky-blue vs blueness

sky-blue and blueness both are nouns.

Word	Noun	Adjective	Verb	Adverb
sky-blue	Yes	Yes	No	No
blueness	Yes	No	No	No

As nouns, blueness is a hypernym of sky-blue; that is, blueness is a word with a broader meaning than sky-blue:

sky-blue: a light shade of blue
blueness: blue color or pigment; resembling the color of the clear sky in the daytime

Other hypernyms of sky-blue include blue.

sky-blue (noun)	blueness (noun)
a light shade of blue	blue color or pigment; resembling the color of the clear sky in the daytime

sky-blue (adjective)	blueness (adjective)
of a deep somewhat purplish blue color similar to that of a clear October sky