purple vs royal purple

purple and royal purple both are nouns.

Word	Noun	Adjective	Verb	Adverb
purple	Yes	Yes	Yes	No
royal purple	Yes	No	No	No

As nouns, royal purple is a hyponym of purple; that is, royal purple is a word with a more specific, narrower meaning than purple:

Other hyponyms of purple include lavender, mauve, reddish purple, reddish blue, violet.

purple (noun)	royal purple (noun)
(Roman Catholic Church) official dress of a cardinal; so named after the Tyrial purple color of the robes	a shade of purple tinged with red
a color between red and blue
(in ancient Rome) position of imperial status

purple (adjective)	royal purple (adjective)
of a color intermediate between red and blue
belonging to or befitting a supreme ruler
excessively elaborate or showily expressed

purple (verb)	royal purple (verb)
color purple
become purple