DMcG: a lot of light is scattered by the rain that doesn't make it into the rainbow. It's scattered at a range of angles such that you can see a lot of it outside the rainbow, but not much inside. This makes the sky outside the rainbow look lighter than the sky inside. Because it's scattered in a fairly random way, you don't get any colours.

Hope that makes sense! If you want something a bit more scientific or mathematical, try the link in my second post.

Liz, it's astonishing, isn't it, how serious grown-up people like us can be cheered up so much by a simple rainbow effect like that? It works on me every time.

Steve