It shows a phenomenon known as "iridescence" that occurs somewhat commonly in thin clouds, or the thin edges of thicker clouds, as is the case here. These colors are caused by diffraction of sunlight as it passes through the cloud. The sun itself is just above the top edge of the picture. While the colors are real, the're quite exaggerated here so they're easily visible. To the naked eye, the glare of the sun would normally render these colors almost, if not completely, invisible. They're more easily visible to the naked eye if you have dark sunglasses on. Even so, at their best they're most often washed out and pale, but they're visible if you just look.

Wait till the sun has disappeared behind the edge of a passing cloud, and look to see if you can catch a glimpse of some of these colors in the edges of the cloud. Quite often, you can see them, and once you do, you'll have seen something beautiful that most people never know about, or bother to notice.