NPOI.XSSF.UserModel.XSSFWorkbook xssfworkbook = new NPOI.XSSF.UserModel.XSSFWorkbook();

XSSFCellStyle rowsStyleColor = (XSSFCellStyle)xssfworkbook.CreateCellStyle();

XSSFColor xssfColor = new XSSFColor();
//根据自己需要设置RGB

byte[] colorRgb = { (byte), (byte), (byte) };

xssfColor.SetRgb(colorRgb);

rowsStyleColor.FillForegroundColorColor = xssfColor;

rowsStyleColor.FillPattern = FillPattern.SolidForeground;

ICellStyle rowsStyleColor = (XSSFCellStyle)xssfworkbook.CreateCellStyle();

rowsStyleColor.Alignment = HorizontalAlignment.Center;

rowsStyleColor.VerticalAlignment = VerticalAlignment.Center;

rowsStyleColor.BorderBottom = BorderStyle.Thin;

rowsStyleColor.BorderLeft = BorderStyle.Thin;

rowsStyleColor.BorderRight = BorderStyle.Thin;

rowsStyleColor.BorderTop = BorderStyle.Thin;

rowsStyleColor.WrapText = true;

//设置背景颜色...

rowsStyleColor.FillForegroundColor = ;

rowsStyleColor.FillPattern = FillPattern.SolidForeground;

((XSSFColor)rowsStyleColor.FillForegroundColorColor).SetRgb(new byte[] { , ,  });

//****************************文件流...
public ActionResult ExportExcelFile()
{
    byte[] streamData = null;

    try
    {

　　　　MemoryStream file = new MemoryStream();

　　　　workbook.Write(file);
　　　　/*这个错误导致我一直以为NPOI不能导出不报错的.xlsx类型的Excel呢!!!···*/

　　　　streamData = file.ToArray();//这种写法就可以的...

　　　　//streamData =file.GetBuffer();//用这么的写法，导出的Excel，就是报错,类似格式不正确，需要修复才能打开...

        if (streamData == null || streamData.Length == 0) { return Content("无数据供导出。"); }

        Response.Charset = "UTF-8";
        Response.ContentEncoding = System.Text.Encoding.GetEncoding("UTF-8");
        Response.ContentType = "application/octet-stream";
        Response.AddHeader("Content-Disposition", "filename=Test.xlsx");
        Response.AddHeader("Content-Length", streamData.LongLength.ToString());
        Response.BinaryWrite(streamData);
        Response.Flush();
        Response.End();

        return null;
    }
    catch (Exception ex)
    {
        return Content("未知错误>>>");
    }
}