iron blue vs Davy's grey

iron blue and Davy's grey both are nouns.

Word	Noun	Adjective	Verb	Adverb
iron blue	Yes	No	No	No
Davy's grey	Yes	No	No	No

As nouns, iron blue and Davy's grey are synonyms defined as:

Other synonyms of iron blue include Davy's gray, steel gray, steel grey.

iron blue (noun)	Davy's grey (noun)
any of various blue pigments	slightly purplish or bluish dark grey
slightly purplish or bluish dark grey