Payne's grey vs Payne's gray

Payne's grey and Payne's gray both are nouns.

Word	Noun	Adjective	Verb	Adverb
Payne's grey	Yes	No	No	No
Payne's gray	Yes	No	No	No

As nouns, Payne's grey and Payne's gray are synonyms defined as:

Payne's grey and Payne's gray: any pigment that produces a greyish to dark greyish blue

Payne's grey (noun)	Payne's gray (noun)
any pigment that produces a greyish to dark greyish blue	any pigment that produces a greyish to dark greyish blue