Exacty. And different courts have come to different conclusions, which is fascinating.
What I was originally trying to get the answer to, though, is why do some artists here get nervous when the "elements of an original" are from a reference such as a photo of David Bowie -- the photo copyrighted by the photographer, Bowie's image owned by himself/now his estate -- but they they don't hold the same reservations when it comes to painting other copyrighted/trademarked material, e.g. the aforementioned candy wrappers, cartoon figurines, board games, etc.
Personally, I wouldn't worry about any of it, because I'm fairly certain it falls into the category of "transformative" as defined by the courts, using the four point test often employed..... But if one does take issue with it, I would think one would take issue with all of it.
Of course, I think the best course of action is to think less about all of this and just paint, paint, paint. And paint some more.