I've got 2 of my Macs on 10.10 a 2012 Air, 2014 Mac Mini. Yet they boot different colours. One with a black screen white Apple, and the other with a white background and a black Apple. Why is this?
It seems to be determined at a hardware/firmware level. Modern Macs all use the black screen (including my 2014 MacBook Air), older ones use the gray screen.
I'm not sure exactly when the cut-off is, but it seems to be different for various product lines. MacBook Pro with Retina display (2012) uses the black screen, for example.