We do try to make much of the UI skinnable.... but we haven't exposed that one yet. In general, you can find settings for different UI features in Section 4.2 of the Component Reference.
It's a good bit of work, but one option would be to use roVideoPlayer and roImageCanvas to draw your own buffer screen. That would allow you not only the possibility of a different color progress bar, but also the ability to place arbitrary text and graphics on your buffer screen.