XAML Button Invisibility with Command Binding

Question

<Button x:Name="StartButton" Style="{DynamicResource StartTimerButton}"
    Width="30.75" Margin="10,0.25,6,0" ToolTip="Start Timer"
    Command="{Binding ExecuteStartStopWatch}" VerticleAlignment="Center" />

<Button x:Name="PauseButton"  Visibility="Collapsed"
    Style="{DynamicResource StartTimerButton}" Width="30.75"
    Margin="10,0.25,6,0" ToolTip="Start Timer"
    Command="{Binding ExecutePauseStopWatch}" VerticleAlignment="Center" >

<Button.Style>
    <Style>
        <Style.Triggers>
            <DataTrigger
                Binding="{Binding ElementName=PauseButton, Path=Button.Visibility}"
                Value="Collapsed" >
                <Setter Property="Button.Visibility" Value="Visible" />
            </DataTrigger>
        </Style.Triggers>
    </Style>
</Button.Style>

I have two different button styles One is Green which is startButton, and the other one is red which is Pause button. When i click the start button it will turn green and bind to StartStopWatch. It then will turn Red and when i click the red button it binds to pausestop watch, but I am not able to get it to accept two styles. Any idea?