Coloured is pretty much anyone who doesn't fall under Black, White or Indian.
The Coloured grouping would include the Cape Malay people, who were originally from areas like Malaysia, Indonesia, etc., 'mixed breed' people (people whose descendants had white/indian/whatever non-black blood).
White women, who had children with non-White fathers could 'change their status' to Coloured so they'd be able to stay with their children. Unless the child was very fair, at which point they could lie or slide some notes under a desk and have the child classified as White and simply say they didn't know who the father was.
People from North Africa like Egyptians, Saudis, etc of course were termed Indian. Unless they were fair skinned enough to be White. The Chinese case is interesting because they were classified as Coloured, not Indian (who were actually officially termed Asian) and people from Japan were classified as White. Read into that whatever logic you may. I assume this is because most 'native' Chinese people were descended from labourers & most of the Japanese population were only 1/2 generations old & thus were more affluent. That many were able to import goods from Japan probably had something to do with it. Heh, those Apartheid-istas they sure weren't big dumb nasty hypocrites, no sir.
A Korean turning up at Joburg International would've caused some supervisors head to implode after attempting classification. Eskimoes would probably have been termed Asian as well, Lord knows what Native Americans would've been called. Aborigines I guess would be black, and Hispanics Coloured, unless they were White Hispanics
I don't think the descendants of the Khoi would have been classed as Coloured, they would almost certainly have been Black.