I would like to say that any and every word is perfectly fine in order to describe someone or something, and the only things stopping you from using a certain description should be circumstance and quality. For example:
"A wealthy black is about as exotic as a shark in the Sahara."
Now, that's a good analogy, so give that sentence a point for quality. Circumstance, however, is a whole other ballgame. Since the main subject of the analogy is "a wealthy black", you might want to watch out. If you publish a work with that sentence, you are going to get hate mail saying that you are a racist. So if you don't want hate mail, you might want to delete it. But remember; that analogy is good, even is the subject of it is not.
P.S.: I'm not a racist. I just used that as an example.