I could be wrong, but I believe that is an example of a
rainbow? They are apparently quite common in some countries.
As the linked article indicates, they require the sun to shine through droplets of water in the atmosphere. We usually have
too many droplets of water in the sky in the UK for rainbows to be visible but that is a moot point anyway, because it is rare to have sunlight here.
Unlikely in the extreme because it would require a one-in-a-thousand day-without too-many-droplets to coincide with the annual-UK-sunny-day!
I suspect that the picture was either PhotoShopped, a rare archive photo, or was taken in another country!