Hi,
> 
> sajolida:
> I see that you changed the background from gray to
> black. Was this on purpose?
> 
Within the current Mac Boot ROM version (2015-001) [0], there are two 
background colors:
     - Gray  = Old (Pre Mavericks/Yosemite)
     - Black = New (Post Mavericks/Yosemite)
I am unable to confirm anything more specific :(
It is up to you which you will use but they are both correct.
Old = Tested on:
     - Old = MacBookPro9,2
     - New = MacBookPro11,3
Wordlife,
Spencer
[0]: 
https://support.apple.com/kb/DL1823