I'm not a fan of silverstone, so guess it may have to be white or blue. I wish they would offer the steel grey as the 4.8is always looked great in grey.
Black would be great, however after all the wear and tear my 4.8is has seen with black, I'm sure the front of the ///M would get trashed in that color. Plus I think black will hide the front bumpers body lines too much (similar issue with my 4.8is).

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'98 Type R drag car ~1400hp
'01 X5 3.0 silver (sold)
'02 X5 4.6is silver (sold)
'04 X5 4.8is black (sold)
'10 X5 ///M Alpine White