Rare Meteorological Phenomenon known as "Cloud Iridescence"

We are all familiar with rainbow and how it is formed. But, have you ever seen a rainbow within clouds? This phenomenon is called cloud iridescence. It is a Greek personification of the rainbow

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How does Cloud Iridescence form?

Cloud iridescence is a diffraction phenomenon caused by small water droplets or small ice crystals individually scattering light. Iridescence is therefore mostly seen at cloud edges or in semi-transparent clouds, while newly forming clouds produce the brightest and most colorful iridescence.

And for you to observe this phenomenon, you need to place the sun itself behind foreground object, a building or a mountain.

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Other aids are dark glasses, or observing the sky reflected in a convex mirror or in the pool of water.

References:
https://en.wikipedia.org/wiki/Cloud_iridescence
http://earthsky.org/earth/i-saw-a-cloud-with-rainbow-colors-what-causes-it