If you set the Office2010Silver theme to RadProgressBar and set the BackColors of the UpperIndicator through the Edit UI Elements dialog, you will notice that your colors are not shown and you still get the colors defined by the theme. It seems that this behavior is determined by the DrawFill property of the UpperIndicator which is false. This is interesting, because if DrawFill is false, the UpperIndicator should not be visible at all.
This behavior is due to the fact that in Office2010 themes the UpperIndicator fill is ImageShape and not a gradient fill.