You can setup a template for the progressbar. Here is some code from a Microsoft example page.
http://msdn.microsoft.com/en-us/magazine/cc163497.aspx#S4[
^]
<ProgressBar Margin="50" HorizontalAlignment="Center"
Value="{Binding ElementName=scroll, Path=Value}">
<ProgressBar.Template>
<ControlTemplate>
<StackPanel>
<Rectangle Name="PART_Track"
Height="20" Width="300" Fill="Red" />
<Rectangle Name="PART_Indicator"
Height="20" Fill="Blue" />
</StackPanel>
</ControlTemplate>
</ProgressBar.Template>
</ProgressBar>