using System.Drawing.Drawing2D;
using DevExpress.XtraTreeList;
// ...

private void treeList1_CustomDrawFooter(object sender, CustomDrawEventArgs e) {
   LinearGradientBrush footerBrush = new LinearGradientBrush(e.Bounds, 
      Color.FromArgb(170, Color.Orange), Color.FromArgb(170, Color.DarkGreen), 
      LinearGradientMode.Vertical);

   using(footerBrush) {
      e.Graphics.FillRectangle(footerBrush, e.Bounds);
   }

   e.Handled = true;
}

Imports System.Drawing.Drawing2D
Imports DevExpress.XtraTreeList
' ...

Private Sub TreeList1_CustomDrawFooter(ByVal sender As Object, ByVal e As CustomDrawEventArgs) _
Handles TreeList1.CustomDrawFooter
   Dim FooterBrush As New LinearGradientBrush(e.Bounds, Color.FromArgb(170, Color.Orange), _
      Color.FromArgb(170, Color.DarkGreen), LinearGradientMode.Vertical)

   e.Graphics.FillRectangle(FooterBrush, e.Bounds)
   FooterBrush.Dispose()

   e.Handled = True
End Sub