French ultramarine vs French blue

French ultramarine and French blue both are nouns.

Word	Noun	Adjective	Verb	Adverb
French ultramarine	Yes	No	No	No
French blue	Yes	No	No	No

As nouns, French ultramarine and French blue are synonyms defined as:

French ultramarine and French blue: ultramarine pigment prepared artificially

Other synonyms of French ultramarine include French ultramarine blue.

French ultramarine (noun)	French blue (noun)
ultramarine pigment prepared artificially	ultramarine pigment prepared artificially