retinal purple vs photopigment

retinal purple and photopigment both are nouns.

Word	Noun	Adjective	Verb	Adverb
retinal purple	Yes	No	No	No
photopigment	Yes	No	No	No

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

retinal purple: a red photopigment in the retinal rods of vertebrates; dissociates into retinene by light
photopigment: a special pigment found in the rods and cones of the retina

retinal purple (noun)	photopigment (noun)
a red photopigment in the retinal rods of vertebrates; dissociates into retinene by light	a special pigment found in the rods and cones of the retina