Silverlight 4 - How can I change button background color when focused with an Implicit button style?

Question

I'm having a great deal of difficulty trying to achieve something that should be trivial. I'm using an Implicit Button Style defined in a global XAML resource file. I just want to change the background color of the focused button to red with a ColorAnimation. I've tried a number of different combinations in Storyboard.TargetProperty and Storyboard.TargetName and nothing has worked. How can I achieve this?

Thanks in advance.

<Style TargetType="Button" >
<Setter Property="Template">
    <Setter.Value>
        <ControlTemplate TargetType="Button">
            <Grid x:Name="grid" RenderTransformOrigin="0.5,0.5">
                <Grid.RenderTransform>
                    <TransformGroup>
                        <ScaleTransform/>
                        <SkewTransform/>
                        <RotateTransform/>
                        <TranslateTransform/>
                    </TransformGroup>
                </Grid.RenderTransform>
                <VisualStateManager.VisualStateGroups>
            <VisualStateGroup x:Name="FocusStates">
            <VisualState x:Name="Focused" >
                <Storyboard>
                    <ColorAnimation Storyboard.TargetProperty="(Rectangle.Fill).(SolidColorBrush.Color)" Storyboard.TargetName="Button" From="Green" To="Red" Duration="00:00:01" />
                </Storyboard>
            </VisualState>
            <VisualState x:Name="Unfocused"/>
            </VisualStateGroup>
            </VisualStateManager.VisualStateGroups>
        ...