sky-blue vs bright blue

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

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

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

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

Other synonyms of sky-blue include azure, cerulean.

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

sky-blue (adjective)	bright 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