It’s an interesting and beautiful atmospheric occurrence, and not terribly common, called “cloud iridescence”, named for Iris, the Greek goddess of the rainbow and messenger of the gods. Here, the sun is behind the larger dark cloud, which makes the cloud appear darker as well as the edges lighter. These cumulus clouds are made of ice and water crystals of about the same size and have to be small for this effect. The angled sunlight shining through the thinning edge is perfectly refracted through them as it is through mist after a rain when we see a rainbow. You can read more about the occurence here.
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