Hello and thank you for posting! 

After reviewing your requirements I have created a sample project for you to demonstrate how the caption and the border background colors could be modified using the default styles for the XamRibbon located at the Infragistics folder: http://es.infragistics.com/community/forums/t/89317.aspx.

In the RibbonWindowContentHost the Grid beckground property could be set to some value like this:

<igRibbon:RibbonWindowBorder BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" Name="WindowBorder" />

                                    <!--<igWindows:CardPanel>-->

                                    <Grid Background="Red">

                                        <Grid.RowDefinitions>

                                            <RowDefinition Height="{Binding Path=ChildDesiredHeight, ElementName=ribbonSize}" />

                                            <RowDefinition Height="*" />

                                            <RowDefinition Height="{Binding Path=ChildDesiredHeight, ElementName=statusSize}" />

                                        </Grid.RowDefinitions>

And in the XamRibbon style the PART_XamRibbonCaption Background property could be changed to some custom value:

<Border Background="Red" CornerRadius="3,3,0,0" Grid.ColumnSpan="3" x:Name="PART_XamRibbonCaption" Height="Auto" />

Please feel free to test the attached project and ask if you have any questions.