Going by the numbers it obviously wasn't popular when they were new, but that isn't to say it isn't now. But like I said I've spent too many years owning silver cars already, so I talked myself into Silver Grey - mainly because of the red interior!
My first Z4 was Ruby Black so I know exactly what you mean about trying to keep black looking smart. I talked myself into that one because of the word "Ruby" and I loved the grey interior.
When less than 600 M Coupes were sold in the UK you could be waiting years to find a specific colour combination!