What happened to people with other skin colors? In Attack on Titan, we know what happened to many people:


   they became Titans.


But what happened to people with different skin colors, e.g. black?


   Did they turn into Caucasian Titans? Or were they simply killed?
This question cannot be answered with canonical information, since it is about a work in progress and the origin story is still shrouded in mystery.

But there are several assumptions you are making:


That the series takes place in Earth.
That even if it is not Earth, there are earth-like ethnics.
That what happened to several people happened to all the people, worldwide. Maybe the titan incident is just restricted to a small continent or a large island, and the rest of the world goes on their medieval life undaunted by the gross titans.
That even if it is in the same Earth as ours, and the titans are worldwide, that It is set in a time where people from several ethnical groups have traveled worldwide, like way after the XIV - XV centuryies great navigations (Columbus, Pedro Álvares Cabral, Vasco da Gama, etc)


So, assuming there are a ethnical pallete in the world of Attack on Titan as broad as our Earth, several hypothesis can be raised:


Asians were included for identification purposes. The target audience of a Japanese manga are Japanese people, so by "placing" them inside the fictional world, it raises identification and acceptance of the work. That is why almost all the 80's and 90's cartoons had a "token kid".



   The first king hand-picked the chosen ones for his walled kindom. He might have intentionally left out the undesirables, as is shown that there is a heavy ethnical bias in place.



The setting is not futuristic. If I had to explain to a layman, I would say "early steampunk". There are gas-powered gadgets, but they are cutting edge tech created on the pressure the survival drive puts on science. Definitely medieval. This was already subject of a Q&amp;A in this site.



   Just as Asians display special characteristics (as do other ethical groups) maybe Africans had some anti-titanification powers or were immune to the Directive.




But your guess is as good as mine. See the disclaimer on top.