navy vs navy blue

navy and navy blue both are nouns.

Word	Noun	Adjective	Verb	Adverb
navy	Yes	No	No	No
navy blue	Yes	No	No	No

As nouns, navy and navy blue are synonyms defined as:

Other synonyms of navy include dark blue.

navy (noun)	navy blue (noun)
a dark shade of blue	a dark shade of blue
an organization of military vessels belonging to a country and available for sea warfare