It's interesting how one thing leads to another and before you know it, a mystery is solved. Last night on the way home from work I saw a spectacular rainbow and as I studied it I noticed narrow bands on the inside of the arc. I checked the Internet and found out that this was a supernumerary rainbow!
Then, as I was perusing the website I clicked on iridescent clouds and this solved a mystery regarding a picture I took in March of 2006 in the Montreal Botanical Gardens.
On that day I had the camera pointed up towards a Goldfinch in a tree when I noticed the strange colouring of some clouds. It was unlike anything I had seen and so I photographed it but never researched it any further. Here is the photograph with reduced brightness to show the colours better. The colours themselves have not been enhanced.