WPF: How do I set a background to default in a key frame of a color animation in XAML?

Question

From this earlier question, I learned that I can set a background to its default color in a ColorAnimation with the following:

<Storyboard>
    <ColorAnimation 
        Storyboard.TargetProperty="(Background).(SolidColorBrush.Color)" 
        Duration="00:00:02.00" 
        To="{x:Null}" />
</Storyboard>

But how do I do this in a key frame? e.g.

<ColorAnimationUsingKeyFrames Storyboard.TargetProperty="(Background).(SolidColorBrush.Color)">
    <LinearColorKeyFrame Value="DEFAULT_COLOR_HERE" KeyTime="0:0:2" />
</ColorAnimationUsingKeyFrames>

Thanks in advanced.