Yosemite Startup screen

Hello:

I have a simple question. I have an iMac (Late 2012: Fully specked with 3 TB Fusion Drive). I just recently noticed that during startup in Yosemite it shows a grey background instead of the black background compare to those Macs who came out with Yosemite out of the box. Is this normal? or It shows some problem with the installation of Yosemite.

Background of the machine and installation of the OS: This iMac fusion drive failed 1 week ago. Good thing it was still covered by Applecare extended warranty. I did a clean installation of Yosemite, and I did a full migration of my data using the Time Machine backup. It wasn't a big deal, but I just noticed that some of the iMac which has a Yosemite installed, every time the machine was booting up or starting up, it shows a black background and a white Apple logo with a progress bar underneath it.

Thank you,

Daniel