I now wonder whether the cause of the problem, oversaturation, is the same for everyone.
I have:
post processing off
shaders and shadows highest option
lower gamma from 1.75 to 1.40
And yes, that is acceptable for me concerning the oversaturation.
In the meanwhile I figured out something else ... monitor ghosting .... solved that too and it's okay for me now so case closed.
Anet asked for input so ......