bright blue vs sky-blue

bright blue is not a noun while sky-blue is a noun.

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

As adjectives, bright blue and sky-blue are synonyms defined as:

bright blue and sky-blue: of a deep somewhat purplish blue color similar to that of a clear October sky

Other synonyms of bright blue include azure, cerulean.

bright blue (noun)	sky-blue (noun)
	a light shade of blue

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