Q:

WPF: Setting Canvas color in a DataTrigger when bound value is null

I'm binding the Background property of a Canvas to some SelectedColorItem object like this:
<Canvas>
    <Canvas.Style>
        <Style TargetType="Canvas">
            <Style.Triggers>
                <DataTrigger Binding="{Binding Path=SelectedColorItem,Mode=OneWay,Converter={StaticResource isNullConverter}}"
                                             Value="True">
                     <Setter Property="Background" Value="Red" />
                 </DataTrigger>
            </Style.Triggers>
        </Style>
    </Canvas.Style>
    <Canvas.Background>
        <SolidColorBrush Color="{Binding SelectedColorItem.MediaColor}" />
    </Canvas.Background>
</Canvas>

It works - it displays the selected color.
What does not work it the DataTrigger which I wrote to indicate that the SelectedColorItem is null.
The simplest isNullValueConverter works either - it might be varified with a Label:
<Label Content="{Binding Path=SelectedColorItem,Mode=OneWay,Converter={StaticResource isNullConverter}}" />

It shows True, when SelectedColorItem is null and False otherwise.
Could somebody tell me what's wrong with the trigger?
P.S.
SelectedColorItem is an instance of a ColorItemClass which contains a MediaColor property:
public System.Windows.Media.Color MediaColor {get;set;}

A:

The thing is that the value set on the element itself has higher "priority" when values from the style, meaning that values set inside the style (including triggers) cannot override locally set values. 
That being set, you need to have both values on the same level, so just move the Background property binding inside the style:
<Canvas>
    <Canvas.Style>
        <Style TargetType="Canvas">
            <Setter Property="Background">
                <Setter.Value>
                    <SolidColorBrush Color="{Binding SelectedColorItem.MediaColor}" />
                </Setter.Value>
            </Setter>
            <Style.Triggers>
                <DataTrigger Binding="{Binding Path=SelectedColorItem,Mode=OneWay,Converter={StaticResource isNullConverter}}"
                                             Value="True">
                     <Setter Property="Background" Value="Red" />
                 </DataTrigger>
            </Style.Triggers>
        </Style>
    </Canvas.Style>
</Canvas>

