I suppose this works as well, the only thing is wouldn't the progress bar still be black and therefore not visible on the black background? Also does that change the initial boot up screen color (before the efi picks a partition to boot from)?
Also isn't your rmbp supposed to have the black boot screen by default in Yosemite?
White progress bar. I believe it should have a black screen by default, so I'm not sure why it was grey instead.