Change KinectTileButton selected color

Question

I am displaying a clipped image (round shape) inside a KinectTileButton. The problem is that I don't know how to change the Selected behavior. By default, when the button gets clicked, the entire area gets darker and I get to see the corners of the square. I want to change the Selected state to transparent so I don't see the corners of the square (as I have a circle inside).

    <k:KinectTileButton Background="Transparent" Width="500" Height="500" Click="ContactClick">
        <Grid Height="500" Width="500">
            <Image Source="/Namespace;component/Resources/Images/Image.png" Width="450" Height="450" x:Name="ContactStream">
                <Image.Clip>
                    <EllipseGeometry RadiusX="220" RadiusY="220" Center="225,225"/>
                </Image.Clip>
            </Image>
        </Grid>
    </k:KinectTileButton>