Generate Excel with merged header using NPOI?

HSSFWorkbook wb = new HSSFWorkbook();
var sheet = wb.CreateSheet("FileName");
var cra = new NPOI.SS.Util.CellRangeAddress(firstRow, lastRow, firstCol, lastCol);
sheet.AddMergedRegion(cra);

Hope help you! (Use lib NPOI)

You need to create two more cells because you have 2x2 merged cells for your headers:

Supplier Provided Data
Deal Provided Data

Then, you'll be able to access your second header cell:

ICell cell1 = sheet.GetRow(0).GetCell(2);

To set the background color, you need to use the FillForegroundColor property:

cellStyleColorGreen.FillPattern = FillPattern.SolidForeground;
cellStyleColorGreen.FillForegroundColor = IndexedColors.Green.Index;

Or to make a custom color:

cellStyleColorCustom.FillPattern = FillPattern.SolidForeground;
((XSSFCellStyle)cellStyleColorCustom).SetFillForegroundColor(new XSSFColor(new byte[] { 198, 239, 206 })); 

To clone a style, for example, to use the cell style for the border and add a background color, you can use:

cellStyleColorAndBorder.CloneStyleFrom(cellStyleBorder);

Full code:

var workbook = new XSSFWorkbook();
var sheet = workbook.CreateSheet("Commission");
var row = sheet.CreateRow(0);

var cellStyleBorder = workbook.CreateCellStyle();
cellStyleBorder.BorderBottom = BorderStyle.Thin;
cellStyleBorder.BorderLeft = BorderStyle.Thin;
cellStyleBorder.BorderRight = BorderStyle.Thin;
cellStyleBorder.BorderTop = BorderStyle.Thin;
cellStyleBorder.Alignment = HorizontalAlignment.Center;
cellStyleBorder.VerticalAlignment = VerticalAlignment.Center;

var cellStyleBorderAndColorGreen = workbook.CreateCellStyle();
cellStyleBorderAndColorGreen.CloneStyleFrom(cellStyleBorder);
cellStyleBorderAndColorGreen.FillPattern = FillPattern.SolidForeground;
((XSSFCellStyle)cellStyleBorderAndColorGreen).SetFillForegroundColor(new XSSFColor(new byte[] { 198, 239, 206 }));

var cellStyleBorderAndColorYellow = workbook.CreateCellStyle();
cellStyleBorderAndColorYellow.CloneStyleFrom(cellStyleBorder);
cellStyleBorderAndColorYellow.FillPattern = FillPattern.SolidForeground;
((XSSFCellStyle)cellStyleBorderAndColorYellow).SetFillForegroundColor(new XSSFColor(new byte[] { 255, 235, 156 }));

row.CreateCell(0);
row.CreateCell(1);
row.CreateCell(2);
row.CreateCell(3);

var r2 = sheet.CreateRow(1);
r2.CreateCell(0, CellType.String).SetCellValue("Name");
r2.Cells[0].CellStyle = cellStyleBorderAndColorGreen;
r2.CreateCell(1, CellType.String).SetCellValue("Address");
r2.Cells[1].CellStyle = cellStyleBorderAndColorGreen;
r2.CreateCell(2, CellType.String).SetCellValue("city");
r2.Cells[2].CellStyle = cellStyleBorderAndColorYellow;
r2.CreateCell(3, CellType.String).SetCellValue("state");
r2.Cells[3].CellStyle = cellStyleBorderAndColorYellow;
var cra = new NPOI.SS.Util.CellRangeAddress(0, 0, 0, 1);
var cra1 = new NPOI.SS.Util.CellRangeAddress(0, 0, 2, 3);
sheet.AddMergedRegion(cra);
sheet.AddMergedRegion(cra1);

ICell cell = sheet.GetRow(0).GetCell(0);
cell.SetCellType(CellType.String);
cell.SetCellValue("Supplier Provided Data");
cell.CellStyle = cellStyleBorderAndColorGreen;
sheet.GetRow(0).GetCell(1).CellStyle = cellStyleBorderAndColorGreen;

ICell cell1 = sheet.GetRow(0).GetCell(2);
cell1.SetCellType(CellType.String);
cell1.SetCellValue("Deal Provided Data");
cell1.CellStyle = cellStyleBorderAndColorYellow;
sheet.GetRow(0).GetCell(3).CellStyle = cellStyleBorderAndColorYellow;

using (FileStream fs = new FileStream(@"c:\temp\excel\test.xlsx", FileMode.Create, FileAccess.Write))
{
    workbook.Write(fs);
}