New Post: LayoutAnchorableControl Title Background Color?

I guess you have to change Background property inside ItemContainerStyle of a LayoutAnchorablePaneControl.

If you go to AvalonDock.Themes.Expression project source code, on Theme.xaml look for style resource named "AvalonDock_Expression_AnchorablePaneControlStyle".
Then look at the Property "ItemContainerStyle":

<Setter Property="ItemContainerStyle">
        <Setter.Value>
            <Style TargetType="{x:Type TabItem}">
                <Setter Property="Background" Value="{DynamicResource AvalonDock_Expression_BaseColor4}"/>
                <Setter Property="Foreground" Value="{DynamicResource AvalonDock_Expression_BaseColor2}"/>
                <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}"/>
                <Setter Property="ToolTip" Value="{Binding ToolTip}"/>
                <Setter Property="Margin" Value="0,0,-10,0"/>
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="{x:Type TabItem}">
                            <Grid SnapsToDevicePixels="true">
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition Width="*"/>
                                    <ColumnDefinition Width="22"/>
                                </Grid.ColumnDefinitions>
                                <Border x:Name="Bd" 
                                      BorderBrush="{TemplateBinding BorderBrush}"
                                      CornerRadius="3,0,0,0"
                                      Background="{TemplateBinding Background}" 
                                      Padding="{TemplateBinding Padding}">
                                    <ContentPresenter 
                                        x:Name="Content" 
                                        ContentSource="Header" 
                                        HorizontalAlignment="{Binding HorizontalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}" 
                                        VerticalAlignment="{Binding VerticalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"
                                        RecognizesAccessKey="True" 
                                        SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                                </Border>

                                <localControls:SplineBorder
                                    Stroke="{TemplateBinding Background}"
                                    Fill="{TemplateBinding Background}" 
                                    Grid.Column="1"/>
                            </Grid>
                            <ControlTemplate.Triggers>
                                <Trigger Property="Selector.IsSelected"
                                    Value="true">
                                        <Setter Property="Background"
                                    Value="{DynamicResource AvalonDock_Expression_BaseColor10}" />
                                        <Setter Property="Panel.ZIndex" 
                                    Value="1" />
                                </Trigger>
                                <Trigger Property="IsEnabled" Value="false">
                                    <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
                                </Trigger>
                                <DataTrigger Binding="{Binding Items.Count, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ItemsControl}}" Value="1">
                                    <Setter Property="IsHitTestVisible" Value="False" />
                                </DataTrigger>
                            </ControlTemplate.Triggers>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
        </Setter.Value>
    </Setter>

Change the "Background" property on line four:

<Setter Property="Background" Value="{DynamicResource AvalonDock_Expression_BaseColor4}"/>

with your desired colour then you got it.

If you wanna change the background of a selected anchor then have a look at the trigger some lines below:

<Trigger Property="Selector.IsSelected"
                                    Value="true">
                                        <Setter Property="Background"
                                    Value="{DynamicResource AvalonDock_Expression_BaseColor10}" />
                                        <Setter Property="Panel.ZIndex" 
                                    Value="1" />
   </Trigger>

Change the "Background" property here.

Hope this helps.