That it's being viewed through a polarized filter is key. If you were to view this unaided, you wouldn't perceive the colours. My guess is that this caused by

That it's being viewed through a polarized filter is key. If you were to view this unaided, you wouldn't perceive the colours. My guess is that this caused by internal reflections in a slew of similarly sized water droplets. Much like a rainbow, except that this is backlit.