The sunset is an illusion; the beauty is real.
It’s just that there’s plenty of empirical evidence to tell us how ocular, neurological and cerebral faculties conspire to turn an external phenomenon into a mental image of shapes and colours, but the perception of beauty itself is an abstract thing which resides in that mysterious place we call consciousness. And that leads me back to one of my favourite suspicions: that a more ‘real’ version of reality is not phenomenal at all, but abstract. I suppose that’s why one leading Buddhist teacher claimed that the most important human endeavour is art.
(And am I not glad that I was always far too mentally lazy to become a philosopher? I am.)