I had a bit of trouble getting this picture. My camera kept thinking everything was too dark and trying to lighten it up, washing out the rainbow in the process. But the rainbow was the whole point!
I think the rainbow is the result of the sunlight coming through the edge of the balcony glass, refracting the light and causing the rainbow. There’s nothing else that I could see that might have caused it. Whatever the source, it’s pretty. I do like rainbows.