How to set the back color for the GroupCaptionPlusMinusCell in the WinForms GridGroupingControl?

this.gridGroupingControl1.TableControlDrawCell += gridGroupingControl1_TableControlDrawCell;

void gridGroupingControl1_TableControlDrawCell(object sender, GridTableControlDrawCellEventArgs e)
{
   GridTableCellStyleInfo style = e.Inner.Style as GridTableCellStyleInfo;

   //Sets the backcolor to the GroupCaptionPlusMinusCell.
   if(style.TableCellIdentity.TableCellType == GridTableCellType.GroupCaptionPlusMinusCell)
   {
      e.Inner.Cancel = true;
      e.Inner.Graphics.FillRectangle(new SolidBrush(Color.Red), e.Inner.Bounds);
      e.Inner.Graphics.DrawRectangle(Pens.Black, e.Inner.Bounds);
      Element el = style.TableCellIdentity.DisplayElement;
      string text = el.ParentGroup.IsExpanded ? "-" : "+";
      Font font = new Font(e.Inner.Style.GdipFont.Name, 10f, FontStyle.Bold);
      int xSpacing = 2, ySpacing = 2;
      if(text == "-")
      {
         xSpacing = 4;
         ySpacing = 1;
      }

      //The plus ad minus(+/-) are printed at the exact position in the cell.
      e.Inner.Graphics.DrawString(text, font, new SolidBrush(Color.White), new Point(e.Inner.Bounds.X + xSpacing, e.Inner.Bounds.Y + ySpacing));
   }
}

AddHandler Me.gridGroupingControl1.TableControlDrawCell, AddressOf gridGroupingControl1_TableControlDrawCell

Private Sub gridGroupingControl1_TableControlDrawCell(ByVal sender As Object, ByVal e As GridTableControlDrawCellEventArgs)
  Dim style As GridTableCellStyleInfo = TryCast(e.Inner.Style, GridTableCellStyleInfo)

  'Sets the backcolor to the GroupCaptionPlusMinusCell.
  If style.TableCellIdentity.TableCellType = GridTableCellType.GroupCaptionPlusMinusCell Then
     e.Inner.Cancel = True
     e.Inner.Graphics.FillRectangle(New SolidBrush(Color.Red), e.Inner.Bounds)
     e.Inner.Graphics.DrawRectangle(Pens.Black, e.Inner.Bounds)
     Dim el As Element = style.TableCellIdentity.DisplayElement
     Dim text As String = If(el.ParentGroup.IsExpanded, "-", "+")
     Dim font As New Font(e.Inner.Style.GdipFont.Name, 10f, FontStyle.Bold)
     Dim xSpacing As Integer = 2, ySpacing As Integer = 2
     If text = "-" Then
        xSpacing = 4
        ySpacing = 1
     End If

     'The plus ad minus(+/-) are printed at the exact position in the cell. 
     e.Inner.Graphics.DrawString(text, font, New SolidBrush(Color.White), New Point(e.Inner.Bounds.X + xSpacing, e.Inner.Bounds.Y + ySpacing))
  End If
End Sub