Again- VISUAL brightness is not the indicator. Not talking about polishing to match a frame, or to look at your gorgeous face in.
Yes, you can polish MIM parts BRIGHTLY. You can polish through the external colors.
What your eye sees isn't the same as what the mating surfaces feel though.

MIMs are more microscopically porous than forged parts.
MIMs don't polish down as slick as forged parts, despite what your eyes see.

You spend an equal amount of time & effort in polishing the engagement surfaces on a MIM and a forged part, stick them under magnification, and you'll see a difference.
Whether you feel it in the trigger pull or not is another matter.
Denis