I personally find it really difficult a) to tell what race someone might be from b) to describe this.
A colleague once called me out on calling someone black, because she was mixed race. To be perfectly honest, I had barely noticed what "colour" she was - it was a group interview and as the colleague didn't know names I referred to her as the "tall black girl" because that was what I remembered about her. If she'd been in front of me, I would have called her "coloured" because she was obviously not "black black" but I didn't remember that at the time as, as I said, I didn't take much notice of it.
I then had a long debate about whether it was OK to refer to her as black, or as coloured. The colleague (white but with very dark-skinned mixed-race partner) was very offended by my opinion that so long as I do not intend to cause offence and try not to cause offence the words I use to describe someone shouldn't matter or be taken as offensive. Which is, quite frankly, ridiculous.
People will refer to you as black, mixed-race, or coloured, because you are not white. White people are obviously white. Black people are obviously black. Mixed-race people, along with Hispanics, Asians and numerous other races, are not so easy to describe. The last thing I, and presumably many others, want to do is to call you something that might offend you. "Black" is therefore often used as shorthand for "anything other than white" and whilst I would always try to reserve black for very dark-skinned people and use coloured/Asian/mixed-race for others, there are issued. Some people take offence at the term coloured, whilst others dislike the alternative "brown". Asian is incredibly generic - people from the Indian subcontinent look very different to those from China, Japan, Korea etc. As for mixed-race, I would only use the term if I knew somebody were mixed-race, and you quite often can't tell that by looking at them. All you can tell is that they are not white.
If the world can agree on acceptable terms to describe certain people, and they are easily identifiable "categories", then I will use them. Otherwise I will continue to use whatever I consider to be the most accurate and least offensive terms I can. And I would hope that anyone ntaking offence would at least have the decency to ask me why I use a certain term before kicking off about it.