Confusing

This photo truly confused me. I thought it would be a black shadow on a purple wall but it seems that purple shadow is what you get if you overexpose it. Can't for the life of me work out what was going on, but I like it anyway... I have a feeling that I should be able to explain it with my newly acquired knowledge on umbras and penumbras (studying for my teaching assessments in primary science).... errr, but can't... Puzzler.