Or, maybe, Arien flirting with Ulmo?
Or then just some water-orientated vala wanted to make beautiful things.
I'm not sure if I've actually read about other rainbows than those of Cirith Ninniach, which probably had no other significance but to make the place look prettier.
However, Melkor seemed to be afraid of all kinds of optical phenomena, so maybe the existence of rainbows might be considered due to the valar trying to scare him. Given also the several meanings of rainbow in this world (peace, hope...), it being a "sign from above" wouldn't sound that far-fetched.
I remember once toying with the idea (I'm not sure if I read it somewhere or invented myself) that the rainbows had something to do with the Straight Road. After all, in several mythologies they were some kind of paths between the dwellings of gods and humans.
Beautiful photos,
alatar!
