So it's OK to say 'negro' about a white person?

I think the best sketch of the movie was the one about "white negroes". It was not a joke on white people who try to behave black. It was a joke on the American politically correct language.

As we know, you cannot use the word "negro" to describe negr..., sorry, black people. But apparently it is OK to use the same taboo word if it applies to white people. Am I getting it right? I don't live in US, so I might not get it.

An alternative theory would be that only neg... black people are allowed to use the word "negro" and it doesn't matter who it applies to.