Sometimes the sunset colors are just crazy. And this day, there was a heavy rain shower just before sunset, so the clouds were still dark and dramatic. When the sun set on the horizon, it lit up all the clouds from below. And I found a good puddle of water near Kulturbroen and set the camera up as close to the water as I could, without getting everything wet.
The image has very little editing, compared to the original RAW file. The sky did look this crazy.