You can be as rude as you like to people as long as it doesn't constitute racial abuse. Put in the same position, ie living in a country where the power is held by people who are discernably different from you and consider themselves superior to you, how would you feel to be on the receiving end of it? Choosing to call people by a name which they have rejected is patronising and done to express contempt. Remember some of the collective names for black people which thankfully we don't hear too often now. Racism usually has a power factor involved and as long as racial groups of people have less power, less access to jobs and services they are the victims of racism however "nice" you are to them. All the groups you refer to are distinct racial groups, but as Einstein says, mixed race is the correct term rather than half-caste.