Brazo,
I have a silver BMW 3-series. Titanium silver, which is probably the same colour as your Z3.
I bought the Souveran for my wifes dark grey metallic Honda, but it will almost certainly end up on my car too at some point!!
Anyway - at the moment I have AG SRP & Wolfgang on the bonnet and boot of my car and I am well impressed with it - really clear and deep looking. This is another product (WG) that is supposed to be ideal for "dark" cars, and I love the look on my silver car. I guess it is all in the eye of the beholder.
I'll let you know how the souveran looks over my car (although I am sure you will try it yourself!)