iron blue vs steel gray

iron blue and steel gray both are nouns.

Word	Noun	Adjective	Verb	Adverb
iron blue	Yes	No	No	No
steel gray	Yes	No	No	No

As nouns, iron blue and steel gray are synonyms defined as:

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

iron blue (noun)	steel gray (noun)
any of various blue pigments	slightly purplish or bluish dark grey
slightly purplish or bluish dark grey