The social construct of whiteness excludes so-called white people from having "colour" or "brownness". These are objectively untrue of people with pale skin, because cream is a colour and a shade of brown. But white supremacists and white culture at large deny objective reality, and substitute their own. This is why the term "people of colour" is able to exist. If it weren't for racists pretending white isn't a colour, it wouldn't make sense. In society, white people are privileged by being treated as though they do not have race. The privilege of being white is never having to think about your race if you don't want to. It's the default. If you say "a worker" or "a politician" or "a firefighter" or "a woman" or "a gay person", society teaches people to think of a white person in all of those cases. Being nonwhite is considered a character trait, and being white isn't. People of colour were often given less depth in older movies, and excluded from being the protagonist, and that's still happening, because society says whiteness is the default. That whiteness... isn't a characteristic. Within this social context, a brown line which is objectively the colour of "white skin", subjectively excludes white people due to their social privilege.