This is an optical phenomenon that is caused by small water droplets or small ice crystals scattering the sunlight by diffraction. If the ice crystals are uniformly larger, then that is when you see that different sly phenomenon of a solar halo or a colourful corona…. I’m always on the lookout for them too :-)
These colourful clouds are usually soft pastel colours as seen in my photographs today; the images were taken with a Conon G15 camera on manual focus on the 12th January 2019. Exposure was 1/2000th second and ISO80.
Have a look at one of my other Astro Blogs of another pretty cloud iridescence and solar corona
that I captured back in 2016 at: