It is an uncommon phenomenon, the result of diffraction ... small droplets of water or small crystals of ice individually scattering light.
If the small droplets/crystals are of similar size, their cumulative effect is seen as iridescent colours. The cloud must be optically thin, so that most rays encounter only a single droplet. Iridescence is therefore mostly seen at cloud edges or in semi-transparent clouds. You may have seen this type of cloud in your own travel, or seen more colourful photos on the internet. But it is so much sweeter if you can capture it with your own camera.
Acripeza reticulata (Mountain Katydid, also known as Mountain Grasshopper) ~~~
The one on left and also the one on top-right is a female. When its wings are folded, it looks like a small lump of dirt and is not easy to be spotted. But when its wings are spread out, you can see the beautiful red, blue & black bands on its body.
The male is much skinnier - bottom-right. Just looking at the photo, you wouldn't have imagined they are the same insect !