roman letters vs roman

roman letters and roman both are nouns.

Word	Noun	Adjective	Verb	Adverb
roman letters	Yes	No	No	No
roman	Yes	No	No	No

As nouns, roman letters and roman are synonyms defined as:

Other synonyms of roman letters include roman print, roman type.

roman letters (noun)	roman (noun)
a typeface used in ancient Roman inscriptions	a typeface used in ancient Roman inscriptions