Hi Francesco,
From what I can see, there's a lot of 'fanciful interpretation' in those images. Some seem clearly 'based on' late Roman shield designs, and some on modern car colours (especially those chequered ones!)..

I suspect none of them are authenric. I see a lot of Osprey influences, and forms from art. The simpler ones (I think!) are the better ones.
The round and oval shield shapes seem basically OK, although I would personally go for a slightly less oval shape.
I don't know about German shield colors or designs, but I can help you with
Late Roman shield designs of the late 4th century from the Notitia Dignitatum. These, although a Medieval copy, give a good idea of the colors and forms found on Roman shields. My guess (a guess only) would be that Germanic shield design would not be very different, although I doubt that there were Germanic 'units' with the same shield colors. I suspect that Germanic warriors had their individual colors, or perhaps smaller groups around a single nobleman had similar shield designs.