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.