What affects the colour of the Startup Screen?
It's based on the board ID in boot.efi. Depending on the model of computer, a black boot screen will be shown (introduced with Yosemite) instead of the older grey screen.

All these Macs and newer show the black boot screen:


MacBookPro10,1 (Retina, Mid 2012)
MacBookAir6,1 (Mid 2013)
iMac14,1 (Late 2013)
MacPro6,1 (Mid 2012)


Here's boot.efi from macOS 10.13.2 with the board IDs highlighted beside the string "Background Color".



You may be able to use Dark Boot to patch your boot.efi's board ID so the dark boot screen is displayed.