What mechanism leads to these spectacular iridescent patterns in clouds labeled as a "Fire Rainbow"? The 21-Feb-2016 BBC new article Singapore 'fire rainbow' cloud phenomenon lights up sky shows several photos of a rainbow-like iridescence in clouds on a sunny day in Singapore (about 1.3 degrees North latitude). Local time was about 17:10, 2 hours before sunset.

I've read about circumhorizontal arcs in this article, but the photos there (and other places) show a definite orientation - approximately parallel to the horizon with red on top (higher elevation angle). 

However the photos from Singapore show tight circular patterns that resemble a color-coded contour map around a hill. So I think the explanation for these would not be identical to the explanation for a classic circumhorizontal arc or "Fire Rainbow". 

below x2: Photo of the phenomenon seen in Singapore, from  Singapore 'fire rainbow' cloud phenomenon lights up sky. Photo credit: Zhou Guang Ping.







below: example of a circumhorizontal arc from Fire Rainbows: A Rare Cloud Phenomenon.
this looks like mother of pearl clouds https://en.wikipedia.org/wiki/Polar_stratospheric_cloud
they are made of ice crystals very high in the atmosphere the only clouds higher in the atmosphere are noctilucent clouds.if you google mother of pearl clouds there is a lot of pictures and information.
as a result of global warming the stratosphere is getting colder this makes
mother of pearl clouds or rainbow clouds visible from larger areas of our planet then before(more info is needed to dokument this statement of mine).

An other alternative is irradecent clouds,They have many of the same properties as mother of pearl clouds https://en.wikipedia.org/wiki/Cloud_iridescence 

The last alternative is a pollen rainbow but they do often form in the direction of the sun or moon(it is a little hard to see if the sun is behind the clouds in your pictures)The colors of a pollen rainbow is often more vibrant than other types of optical phnomena https://soranews24.com/2019/03/09/high-pollen-levels-in-eastern-japan-create-pretty-rainbow-suns/