I was driving west towards Jülich when I saw a silver ball in the sky. The ball flew at very high speed, leaving no trails or similar things in the sky.There were no positioning lights, wings, propellers, turbines etc. visible. I could follow it for a few minutes as it flew into the same direction until I lost it. When I reached the street leading from Welldorf to Stetternich I discovered a strange phenomenon in the clouds (on the western side of the street). Close to a water tower there was a rectangular patch in the clouds which was colored like a rainbow. There was no rain and nothing else of a rainbow visible, only this patch in the clouds. (It does not necesseraly mean that both things relate to each other, but the ball flew towards this patch)