I confirm that on my black 3rd series B20 wheels are unicolor, and a very whitish beige.
My car was restored for a connoisseur beginning of the 90ies, and the car was done right, with expert advice from the Registro Aurelia.
It is not currently at home, so I cannot check the approximate color of the wheel with a RAL-chart.
The photo will have to suffice for now!
I can also show photos of unicolored cream 4th series B20 wheels (generally on blue celeste cars)
And unicolored 4th series wheels in the same color as the body (a "testa di moro" car)
And then there are unicolored wheels with chrome trim rings, also very nice.
So basically, there seems to be an esthetic element to answer that question, and one should decide according to the color of the body!