This is a digital representation of a real phenomenon!
It's called a 'reflection rainbow' and it's produced in between primary and secondary rainbows by sunlight beaming upwards after reflection from calm water or wet sand. Pretty rare and very cool!
If you're interested in learning more about this and other rare rainbow phenomenon, this picture's artist wrote a great tutorial in their Skywatcher Guide :) Click here to see a larger version of this picture. Happy rainbow gazing!