Oddly, I think you're onto something. It's black on my 2013 nMac Pro6,1, white on our oldest mac (a 2011 27" iMac), and black on our new Mac Mini.
No other correlations come to mind.
Well I corrected myself somewhere not long ago. The 2012 models still have grey/white boot screen, so black goes only for "current" laptops, 2013+, although that retina above me seems to be some kind of exception.
2013 15 retina.
If i boot from the internal drive, black.
If i boot from external, grey.
Probably because an OSX on external HDD is not not considered top priority configuration, unlike laptops which still have warranty and are first in the line for fixing issues and bugs, that is all MacBooks 2013+ with OS installed on Apple's default internal storage.
Again, like I said before, grey screen it's probably Apple's way telling us that they consider these devices to be "not THAT important" when in comes to development. Harsh, but understandable.