I don't have a clue to answer your question, but wanted to say "I like it."

Most likely that is a reaction to weather and not normal coloration. It is very pretty. My tropicals always die in my greenhouse so I have no idea if they'd die that nicely outdoors...

I have a yellow water lily that sprouts green leaves but as they mature they turn first mixed green and burgundy before turning full burgundy much like those in your picture.

Just a guess!