I think I saw something that could be related to this last weekend.
I was looking across the valley from our house with the sun behind me and broken cloud. There was a misty cloud passing across in front and brushing the upper part of the hill, but just diffuse enough for you to make out some detail through it.
There were deffinate tints to the colours of the fields from time to time as the sun behind me was exposed intermitantly, but no actual rainbow and the areas of tint looked too big for a conventional rainbow anyway.