Try this please:
Excel.Application xl = new Excel.ApplicationClass();
Excel.Workbook wb = xl.Workbooks.Add(Excel.XlWBATemplate.xlWBATWorkshe et);
Excel.Worksheet ws = (Excel.Worksheet)wb.ActiveSheet;
ws.Cells[1,1] = "Testing";
Excel.Range range = ws.get_Range(ws.Cells[1,1],ws.Cells[1,2]);
range.Merge(true);
range.Interior.ColorIndex =36;
xl.Visible =true;