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Such iridescent clouds appear due to diffraction effects if the clouds water droplets are of similar sizes. Due to the different particle size distribution inside the clouds the colors often follow the clouds shape. But very close to the sun the angular distance to it becomes much more important and such iridescence evolves to a so called Aureole or Corona with concentric rings.
M.G.J. Minnaert: "Light and Color in the Outdoors", Springer-Verlag
R. Greenler: "Rainbows, Halos, and Glories", Cambridge University Press
L. Cowley: "Atmospheric Optics" web site