MLA

Shelley, Samuel, approximately 1750-1808, artist. Unknown gentleman in gray coat Unknown gentleman in red coat. [art original]. [18th century?]. https://collections.library.yale.edu/catalog/15815230.

APA, 6th edition

Shelley, Samuel, approximately 1750-1808, artist. ([18th century?])[Unknown gentleman in gray coat Unknown gentleman in red coat. [art original]]. https://collections.library.yale.edu/catalog/15815230.