So you have summaries in the group caption row? If so, you can try using QueryCellStyleInfo to color the different levels. Here is code that colors level 1.
private void gridGroupingControl1_QueryCellStyleInfo(object sender, GridTableCellStyleInfoEventArgs e)
{
if(e.TableCellIdentity.TableCellType == GridTableCellType.GroupCaptionSummaryCell)
{
GridCaptionRow caption = e.TableCellIdentity.DisplayElement as GridCaptionRow;
if(caption.ParentGroup.GroupLevel == 1)
e.Style.BackColor = Color.LightGoldenrodYellow;
}
}