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!".
I would bet at least 100$ against 20$ that it is in fact what she is saying, intentionally. It's not the first time that she packs information into such small details; e.g. you can infer the date at which the books are taking place from the date of death of the headless nick and his 1000th celebration thing - and that date was stated once in the very first book. The series has an amazing attention to detail.
Why does anyone care? It's a crappy continuation of a mediocre series, it's not like you're actually going to see the play or anything.
I think the harry potter series is a masterpiece - that's why I care; otherwise I wouldn't.
It's an issue if you're saying ethnicity Y people cannot play roles based on source material where the base character had ethnicity Z. Especially when the relative sizes of Y, Z, etc. have great disparity.
I'm saying that if a black person plays a white person in a stage play or vice versa then you're being inauthentic because they look different. Other assets, cultural, whatever, different story, because you can't see them. They're actors, they play a character.
