CentOS - meaning of the boot screen progress bar layers/colors

I'm not an expert, but from the source (ply-text-progress-bar.c) it looks like you set the overall percentage done and that the different colors/layers's progress is hard coded by the following, and other functions within that file:

    brown_fraction = -(progress_bar->percent_done * progress_bar->percent_done) + 2 * progress_bar->percent_done;
    blue_fraction = progress_bar->percent_done;
    white_fraction = progress_bar->percent_done * progress_bar->percent_done;

So, it appears that it is a style choice with no underlying meaning.

The software that is running to produce that bar is called Plymouth, and it only appears if you have rhgb in your kernel arguments. It basically takes the output of the startup sequence and presents it according to the chosen theme. In this case, it is using the text theme, probably because it can't start the graphical theme.