How to change the Header BackColor of a Stacked WinForms GroupBar when it is in Office2003 Visual Style?
Visual style
In GroupBar, you can customize the header color of the GroupBarItem by using the class named Office2003Colors.
C#
//Sets the Office2003 visual style
this.groupBar1.VisualStyle = Syncfusion.Windows.Forms.VisualStyle.Office2003;
//Enables stacked mode
groupBar1.StackedMode = true;
//Customizes the Header BackColor of GroupBarItem's
Syncfusion.Windows.Forms.Office2003Colors.GroupBarHeaderColorDark = Color.Red;
Syncfusion.Windows.Forms.Office2003Colors.GroupBarHeaderColorLight = Color.White;
VB
'Sets the Office2003 visual style
Me.groupBar1.VisualStyle = Syncfusion.Windows.Forms.VisualStyle.Office2003
'Enables stacked mode
groupBar1.StackedMode = True
'Customizes the Header BackColor of GroupBarItem's
Syncfusion.Windows.Forms.Office2003Colors.GroupBarHeaderColorDark = Color.Red
Syncfusion.Windows.Forms.Office2003Colors.GroupBarHeaderColorLight = Color.White
Note:
In GroupBar, StackedMode property is enabled to customize the appearance of the GroupBar header.

Figure 1: GroupBar header backcolor customization in Office2003 visual style
Samples:
C#: http://www.syncfusion.com/downloads/support/directtrac/139533/GroupBar1603706498.zip
VB: http://www.syncfusion.com/downloads/support/directtrac/general/GroupBar_VB-267415748.zip