royal purple vs purple

royal purple and purple both are nouns.

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

As nouns, purple is a hypernym of royal purple; that is, purple is a word with a broader meaning than royal purple:

Other hypernyms of royal purple include purpleness.

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

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

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