R is for rainbow clouds

It was on vacation in Tuscany. We were sitting on a staircase and enjoyed the sun as my friend suddenly said: "Look at the clouds!"
They were ... colorful. I quickly grabbed the camera (which is on holiday so thank God always with me) and shot many many photos. After a few minutes the clouds were white again.
I was completely intrigued and researched at home what this phenomenon was.
It must have to do with small ice crystals in the clouds. Obviously, this phenomenon rarely observed and then in more northern areas. After that I saw one or two times rainbow clouds in Germany, but never in such an extreme way.