Okay, to end this discussion about what skin color she really has - it actually is stated in the books.
Harry Potter and the Prisoner of Azkaban, chapter 21 : "Hermione's white face was sticking out from behind a tree."
(See? Subtle! I'm sure this has been intentional.)
I'm going to assume this is where they're running from werewolves and dementors. I could easily see someone describing someone's face as white if you're being chased by crazy stuff. BUT I think this shows that Rowling thought of Hermione as light skinned. But I don't think it's her intentional way of saying "she's definately white, guys! I'll point to this later!".
There's also this at the beginning of Azkaban:
They were there, both of them, sitting outside Florean Fortescue’s Ice Cream Parlor — Ron looking incredibly freckly, Hermione very brown, both waving frantically at him.
And definition 5 of "white" from dictionary.com:
pallid or pale, as from fear or other strong emotion: white with rage.
This fits the context of the scene where that description is used.
This outrage just reminds me of all the people complaining that Rue in Hunger Games was played by a black girl even though she was explicitly described as such in the books. It's a
little more understandable here since Hermione's race is ambiguous, but still.
To add fuel to the fire: Earthsea.
Is that an example of casting separate from source race? Morgan Freeman in The Shawshank Redemption is an example. In the novel, his character was specifically and unambiguously stated to be Irish (like, white-skin, red-head Irish, hence the name, 'Red'). The movie makes a reference to this (there is a line by Freeman, "Maybe because I'm Irish" about his name), which I always thought was extremely clever.
Also, one of the best (or just the best?) adaptations of a book to screen.
Almost all (or actually all?) of the main characters in Earthsea are black. This was not necessarily important to the plot (I read it long ago) but it
was important to the author, who noted the near-complete lack of POC main characters in fantasy and sci-fi and strove to do better in her work.
IIRC, a TV mini-series for Earthsea was produced and pretty much the entire cast was white-washed.
Maybe they could argue that the actors were the right ones for the job, except they were kind of bland and terrible? I'm not sure, I did not watch the series and I'm only half-remembering the discussion from when this was current news.
Regarding the point about Morgan Freeman, the problem is that Hollywood has a long history of casting white actors to play POC. Some examples that come to mind are The Last Airbender and pretty much every single Hollywood film set in ancient Egypt. When this happens and POC complain, the overwhelming response tends to be "it's an interpretation" or "they're just the best actor for the role", etc. When the reverse happens and a non-white actor is cast in a role traditionally perceived as white, those same people freak out. And note that this is far less common.
