This week, there was a school shooting in Canada (Montreal) and the shooter was a 25-year-old Canadian man. Hold on...what image did you get when I said "Canadian man"? Did you imagine a White guy? Most likely you did, because many people assume "Canadian = White".
The Dawson College killer was Kimveer Gill, which is a name that could be interpreted as mixed-race because it sounds ethnically unclear. Shortly after the shooting, a witness was being interviewed and he was asked what race the shooter was. He shrugged and said "White"...he said it as if it didn't matter what race the killer was.
It turns out Gill's heritage is Indian (from India) and he was born in Canada. He posted some pictures of himself on the Internet (which have since been taken down), and he does look somewhat mixed-race. However, he is technically Asian (because India's in Asia), one witness said he was White, and on one of his photos, someone made a comment calling him a Middle Eastern terrorist.
So...a witness thought he was White, an angry person on the Internet thought he's Arabic, but in reality, he's Asian. The shooting is believed to be random violence, and not a racist or terrorist attack. Yet when people see what he looks like or learn what his ethnicity is, some of them immediately make racist comments. Some of them start thinking it WAS racist or terrorist, even when the attack itself wasn't.
All of this shows that people usually assume that individual humans can only belong to one race. The witness who said that he was White, was responding to a question of whether Gill was "White, Black, (or) Asian". White, Black, Asian? You mean there's no in-between? Of course there is! Now, let's say Kimveer Gill WAS mixed-race instead of only Indian. Let's say he was White, Black, AND Asian (yes, that's genetically possible). Would there still be racist comments towards him? There definitely would be, but what?