Last night at sunset I saw a beautiful iridescent cloud, the best example I have ever witnessed.
The photo was taken from my mobile phone and gives no idea at all really, but I hope you can just about see that the shiny cloud in the middle displays a range of bright pastel colours, rather like mother-of-pearl.
These clouds are quite rare. They are caused by sunlight shining through clouds whose water droplets (or ice crystals) are all the same size in any small part of the cloud but vary in size between one part of the cloud and another. They act a bit like an oil-slick on a puddle.
Atmospheric optics are an interest of mine. The most familiar example is a rainbow, but there are many others. Sun dogs are commoner than rainbows, but less conspicuous so many people have never noticed one. But as soon as you become aware of them, perhaps just from a single example, you learn to recognize the cloud conditions when it's worth trying to spot a sun dog.
Here is a gallery of remarkable photos of iridescent clouds.