Hi there In the modern U.K. this would certainly be seen as racist and offensive. Better to refer either to their clothing, running number or such like.

Yes, it is offensive, but descriptive. Today, at least in the United States everyone is afraid of saying anything regarding somone's skin color.

This is a difficult question to answer. Some people might find it offensive, but I personally would not. Referring to any non-white by their race can be offensive, so if you're talking to strangers it's best to use other descriptions such as their clothes or their height, etc.

It is personal to say, if some people really care about their skin colour, had better not to mention it no matter what skin colour is.

I don't know, maybe "people of colour" can be less ofensive. I read forums and there are different thoughts. In spanish I prefer to say "moreno" instead of "negro" although " moreno" is every non-white people. I don't know if there is a Soft english word / term for that.