Dialog and Menu Background color transparency

[Fonte Boa]

I noticed in Preferences|User Interface the option "Use the OS native color dialog".
When i ticked it, close Preferences window and open it again, i can see native Windows color dialog. And if i do it again, i can see another variation (not Windows scheme).

Is this other one native Mac OS?
Or is it a Wings specifc one? (if so, this would be the ideal window to add Alpha slider).