iron blue vs steel grey

iron blue and steel grey both are nouns.

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

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

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

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