User blog comment:B214/High School DxD BorN episode 1 review/@comment-3965848-20150407194754

From what I saw from the episode, the sky in the Underworld is green which I'm guessing is day time, while last we saw it was purple which must be when it's night time.

Although I can't remember when this happened or where I read it, but I think they used magic to change the night sky similer to ours, even created thier own moon.