bright blue vs cerulean

bright blue is not a noun while cerulean is a noun.

Word	Noun	Adjective	Verb	Adverb
bright blue	No	Yes	No	No
cerulean	Yes	Yes	No	No

As adjectives, bright blue and cerulean are synonyms defined as:

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

Other synonyms of bright blue include azure, sky-blue.

bright blue (noun)	cerulean (noun)
	a light shade of blue

bright blue (adjective)	cerulean (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