网格中的WPF MVVM组合框改变单个背景

<Grid>
    <ComboBox SelectedValue="{Binding SelectedItem}" Width="100" Height="30">
        <ComboBoxItem Content="Red" Tag="Red"/>
        <ComboBoxItem Content="Blue" Tag="Blue"/>
        <ComboBoxItem Content="Green" Tag="Green"/>
    </ComboBox>
    <Grid.Style>
        <Style TargetType="Grid">
            <Style.Triggers>
                <DataTrigger Binding="{Binding SelectedItem}" Value="Red">
                    <Setter Property="Background" Value="Red"/>
                </DataTrigger>
                <DataTrigger Binding="{Binding SelectedItem}" Value="Blue">
                    <Setter Property="Background" Value="Blue"/>
                </DataTrigger>
                <DataTrigger Binding="{Binding SelectedItem}" Value="Green">
                    <Setter Property="Background" Value="Green"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </Grid.Style>
</Grid>