You cannot directly set the "Foreground" property of your control using a ColorAnimation. You can, however, set the color of its Foreground brush by explicitly setting the brush in XAML and giving it a name:
<ProgressBar x:Name="ProgressBar" Grid.Row="1" VerticalAlignment="Bottom" Value="{TemplateBinding Progress}">
<ProgressBar.Foreground>
<SolidColorBrush x:Name="ProgressBrush" Color="Orange"/>
</ProgressBar.Foreground>
Now that you have a reference to the control's Foreground brush property ("ProgressBrush") you can animate it's color using the Color Animation.
<ColorAnimation Storyboard.TargetName="ProgressBrush" Storyboard.TargetProperty="Color"
To="Black" Duration="0:0:0.25" EnableDependentAnimation="True"/>
Hope this helps! Reference: How to: Animate the Color or Opacity of a SolidColorBrush