Please try this:
<DataTemplate DataType="{x:Type local:Car}">
<Rectangle Width="100" Height="100" Fill="Red"
Tag="{Binding RelativeSource={RelativeSource AncestorType={x:Type Window}}}">
<Rectangle.ContextMenu>
<ContextMenu>
<MenuItem Header="{Binding Colour}"/>
<MenuItem Header="{Binding Path=PlacementTarget.Tag.ActualWidth, RelativeSource={RelativeSource AncestorType=ContextMenu}}"/>
</ContextMenu>
</Rectangle.ContextMenu>
</Rectangle>
</DataTemplate>
See my answer here too.