procedure TForm1.VirtualStringTree1AdvancedHeaderDraw(Sender: TVTHeader; var PaintInfo: THeaderPaintInfo;
  const Elements: THeaderPaintElements);
begin
  if hpeBackground in Elements then
  begin
    PaintInfo.TargetCanvas.Brush.Color := clGray;
    PaintInfo.TargetCanvas.FillRect(PaintInfo.PaintRectangle);
  end;
end;

procedure TForm1.VirtualStringTree1HeaderDrawQueryElements(Sender: TVTHeader; var PaintInfo: THeaderPaintInfo;
  var Elements: THeaderPaintElements);
begin
  // if there is no column assigned in the paint info, elements for the background painting are requested;
  // this event fires later for plates as well, but we want to paint only the background by ourselves
  if not Assigned(PaintInfo.Column) then
    Elements := [hpeBackground];
end;