Living as I do in France, where (from what I see) 90% of the cars on the road are in shades of grey or silver, I have to confess that grey doesn't exactly float my boat. Black does of course show the dirt, but with a car like a Z4, that is likely to be regularly cleaned and pampered, that is not a major issue. You do, of course, have Ruby and Saphire options even if you choose a Black Z4, and the subtle colour shift in certain lights is very pleasing - although if you have a scape, will probably cost more to repaint.
Scorp.
Biased Ruby Black driver.