This is the cover of the Rainbow album Down to Earth, on which their recording of “Since You Been Gone” appears. See anything wrong with that? How about a rainbow in outer space! As far as I can tell, you need liquid water to make a rainbow and in outer space, that liquid water would just boil off into the vacuum. I guess that’s just how metal Rainbow was, though: so metal that the laws of physics didn’t apply to them!
A more cynical viewpoint might be that the audience isn’t smart enough to know the difference, but we know that at least some rock stars are pretty smart cookies. Besides, it’s a good idea to never underestimate your audience. They’re generally sensitive, intelligent people who respond positively to quality entertainment. And yes, that’s a quote that I stole from this scene from the excellent 200th episode of “Stargate SG-1” where they lampoon lots of popular sci-fi tomes and franchises. It’s just a shame that Rainbow couldn’t have had a cameo in there to clarify that the first explanation was in fact the one they had in mind, because “too metal for physics” just sounds cool.