While I have nothing but respect for Greta Thunberg and her multiple causes, we must not forget she is primarily a public and political figure. What we see, what she says, is carefully crafted to serve those causes. While I don't doubt for a second she is Autistic, she could very well uses this as a way to say and do things she otherwise could not.
Masking is a form of protection, but it can also be a tool, or even a weapon. I'm convinced the Greta we see is a persona. Very close to the real her, but some things hidden and some exaggerated. That "black and white" thing might be more a communication tool than a real trait of her.