Turner was tasked by Dr. Monro to do the watercoloring of the copies, while Thomas Girtin did the pencilling.
Referring to a copy of John Robert Cozzens, Barry Venning remarks,
[i]Turner’s washed in effects…do not follow Cozens very closely–he reverses the light and shadowed areas in the original.[/i]
This is an indication that even as a copyist, Turner wasn’t content to merely reproduce what was before him but that he was already intent on going his own way.
This is the Cozens referred to: