Assuming you are talking about the rear 'diffuser' or the plastic trim that surrounds the exhaust tips and spans the width of the rear bumper, YES, it will be a slightly different color on M Sport versions. All US Spec M Sport trimmed 3 series come with a dark grey 'diffuser'. However on the MG Metallic paint it is very close in color, while on the lighter colored M Sport cars the contrast is readily apparent.
Technically all US Spec 3 series cars have body colored rear bumper skins. It's just the 'diffuser' that contrasts on the M Sport option, the rest of the bumper still matches the overall car color.
You can easily see this on any MG picture thread or the contrast is pretty obvious if you go to BMWUSA.com and build the car and look at the rear view.