I was in the mountains of the National Parc of Gaspésie, at about 1000 m altitude. I walked through a gray fog to the top. At the summit, the sky opened up to show a white rainbow, a rainbow of ice ?
I took a photo once with the same phenomenon and without ice. I have read that the fog has finer droplets than the rain : that's why the light through it can't diffract and the consequence is that the light stay white !
That day, I felt in paradise !
November 2012, Parc de La Gaspésie, Québec, Canada