iron-grey vs achromatic

iron-grey is a noun but achromatic is not a noun.

Word	Noun	Adjective	Verb	Adverb
iron-grey	Yes	Yes	No	No
achromatic	No	Yes	No	No

iron-grey (noun)	achromatic (noun)
the color of freshly broken cast iron

iron-grey (adjective)	achromatic (adjective)
of the grey color of iron	having no hue