purple milk vetch vs milk vetch

purple milk vetch and milk vetch both are nouns.

Word	Noun	Adjective	Verb	Adverb
purple milk vetch	Yes	No	No	No
milk vetch	Yes	No	No	No

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

purple milk vetch: perennial of southern and western Europe having dense racemes of purple or violet flowers
milk vetch: any of various plants of the genus Astragalus

Other hypernyms of purple milk vetch include milk-vetch.

purple milk vetch (noun)	milk vetch (noun)
perennial of southern and western Europe having dense racemes of purple or violet flowers	any of various plants of the genus Astragalus