Iridescent Pileus Cloud ~ Imaged by Alana Drew at Vero Beach, Florida. ©Alana Drew, shown with permission.

Pileus or 'cap' clouds are thin, freshly formed and with uniform sized droplets. They are ideal to produce bright iridescent colours by diffraction.
Atmospheric
Optics

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A cumulus cloud containing warm air boils, convects, upwards.

Air layers above the cloud are also pushed upwards by the growing and rising cumulus.

Rising air expands and adiabatically cools .
Sometimes there is a thin layer of humid air above the cloud. As that layer is forced upwards it expands and cools. Water vapour in it condenses into droplets. The resulting thin cloud layer sits as a cap or pileus over the rising cumulus.

The bright iridescent colours seen in pileus are diffracted sunlight. Iridescent colours are strongest when the diffracting droplets are small and similar in size. The newly formed pileus droplet all of similar provenance are ideal for iridescence.