Please help: Half caste poetry?

The poet is creating images that mirror a satirical comment on the concept of being "half-caste", or "mixed blood", or "half black". He's asking rhetorical questions that indirectly show the idiocy of the concept, revealing that nothing is "half", it's always "whole", and does so by recounting famous artists and common images (such as the English sky). Read the poem again with this in mind and your answer will be self-evident.