navy vs dark blue

navy and dark blue both are nouns.

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

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

Other synonyms of navy include navy blue.

navy (noun)	dark 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