cerulean vs bright blue

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

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

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

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

Other synonyms of cerulean include azure, sky-blue.

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

cerulean (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