From the Rectangle doc

To paint the inside of the rectangle, set its Fill property to a Brush-derived object.

So you may use Fill property instead of BackgroundColor,

            <Rectangle.Triggers>
                <DataTrigger TargetType="Rectangle" Binding="{Binding Cancelled}" Value="True">
                    <Setter Property="Fill" Value="#FF0000" />
                </DataTrigger>
            </Rectangle.Triggers> 

Please let me know if you have any question.