It's important to realise that we live in a society where whiteness is assumed to be the "default". This is why when JK Rowling portrays characters who aren't white, it's necessary for her to explicitly specify this (e.g. Blaise Zabini). If JKR were to specify that a character was white (e.g. "Harry was an 11 year old white boy with black hair"), it would feel awkward, because our society always assumes whiteness unless there is evidence to the contrary.
Thandeka said that if her luncheon partner played The Race Game for one week, she would then answer the colleague’s question. The game consisted of one rule. For one week the woman was to “use the ascriptive term white” whenever she mentioned the names of “Euro-American cohorts. She must say, for instance, ‘my white husband, Phil,’ or ‘my white friend Julie,’ or ‘my lovely white child Jackie.’” The women never had lunch with Thandeka again.
The question asked for evidence of Hermione's skin color. I would argue that every time JKR mentions Hermione counts as evidence for Hermione being white, because Western civilization and the English language automatically assumes whiteness unless otherwise specified. To quote from one of the articles Richard linked to in his question:
JK Rowling included characters of color in this series—if a student at Hogwarts had brown skin, we knew about it. Lee Jordan and Angelina Johnson? Black. Padma & Pavarti Patel? South Asian. Cho Chang? Nondescript Asian (for a full analysis of Cho Chang and Asian stereotyping see this post by Diana Lee). White characters were not described by race, but by their lack of description.
And again, from another article linked to by Richard:
There's an unspoken rule in literature: Characters are white unless explicitly described otherwise.
It's not an ignorant assumption to assume that Hermione is white, in fact, it shows a sophisticated understanding of how the English language works.