WPF using a dynamic resource inside a data trigger that is inside a style

Question

I have a UserControl that contains a color object as resource:

  <UserControl.Resources>
    <ResourceDictionary>
      <ResourceDictionary.MergedDictionaries>
        ...
      </ResourceDictionary.MergedDictionaries>
      <Color x:Key="BackgroundColor">Transparent</Color>
    </ResourceDictionary>
  </UserControl.Resources>

There is a Grid as root element in this UserControl, that has a custom style in which I would like to reference the color resource above:

    <Grid.Style>
      <Style TargetType="Grid">
        <Style.Triggers>
          <Trigger Property="FrameworkElement.IsMouseOver" Value="True">
            <Trigger.EnterActions>
              <BeginStoryboard>
                <Storyboard>
                  <ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Background).(SolidColorBrush.Color)">
                    <SplineColorKeyFrame KeyTime="0:0:0.5" KeySpline="0.15,0.8 0.3,1" Value="LightBlue" />
                  </ColorAnimationUsingKeyFrames>
                </Storyboard>
              </BeginStoryboard>
            </Trigger.EnterActions>
            <Trigger.ExitActions>
              <BeginStoryboard>
                <Storyboard>

                  <!-- Here is the problem -->
                  <ColorAnimation To="{DynamicResource BackgroundColor}" Duration="0:0:1" Storyboard.TargetProperty="(Background).(SolidColorBrush.Color)" />

                </Storyboard>
              </BeginStoryboard>
            </Trigger.ExitActions>
          </Trigger>
        </Style.Triggers>
      </Style>
    </Grid.Style>

Each time I run the app I get a XAML parse exception when the framework tries to set the style. If I reference the BackgroundColor as StaticResource it works just fine, but I'm changing the resource in code behind at certain conditions so it has to be referenced dynamically.

How can I use the resource as a dynamic resource in this context?