The white or black refers to his parents or ancestors, so he could be white or black or a mixture of skin colours.
After "he could be a mixture of skin colours", "he is a mixture of skin colours" is correct, isn't it? But would it mean?No, 1 makes no sense -- a person cannot be a mixture of skins.
Biracial or multiracial sounds very decent and formal, but less colloquial. Is there a colloquial way to express the same idea with sufficient decency?I would not say someone is a mix. It sounds rather offensive, because it's something you might call a dog. You'd be better off with saying someone is biracial or multiracial.