There have been photos and video this week of gorgeous "rainbow clouds" from the Philippines and Singapore. Conditions combined to give a misty cap cloud above dramatic shower clouds and with sunlight just at the right angle, the colours appeared.
Cumulus clouds build upwards, growing as we often see before April showers in the strong, spring sunshine. This building pushes layers of moist air up above the Cu clouds, which then cools and condenses and can form a cap on the cloud or a Pileus. (Latin for cap)
It is this misty capping cloud which catches the sunlight and gave a dramatic look to some of the clouds seen this week, especially set against the darkening skies.
An accessory cloud of small horizontal extent, in the form of a cap or hood above the top or attached to the upper part of a cumuliform cloud that often penetrates it. Several pileus may fairly often be observed in superposition.
This quick formation of the mist/cap cloud means the small water droplets are quite uniform, and these are ideal for iridescence; diffraction of sunlight giving beautiful colours and so the rainbow clouds. The sunlight is dispersed by the mist droplets into the various colours.
Other rainbow clouds- Circumhorizon arc
Also, know as Circumhorizontal Arc. When the sun is very high in the sky, around noon, mid-summer, equatorial areas. Need a Halo, a very large one, which is an optical phenomenon circle formed by ice crystals in very high cloud. It's so large you can only see a portion, which can look like a nearly straight line or slight arc. The Cirrus cloud takes on the rainbow colours which can be quite brilliant. (Previously known as the lower circumzenithal arc). Also, gets referred to as a 'fire rainbow' but this is not a correct term.
Nacreous Clouds- which caused some excitement in Feb 2016 here in the UK with rare sightings for a few mornings.