how to set background color of a cell using apache pio 4.1.0

Cell interior uses pattern fills. The fill background color is the color behind the pattern.The fill foreground color is the color of the pattern.

To fill the cell using a plain color, you need using fill foreground color and solid pattern.

See Fills and colors.

...
cellStyle.setFillForegroundColor(IndexedColors.GREY_25_PERCENT.getIndex());
cellStyle.setFillPattern(FillPatternType.SOLID_FOREGROUND);
...

Complete example having cell fill and cell content:

import java.io.FileOutputStream;

import org.apache.poi.ss.usermodel.*;
import org.apache.poi.hssf.usermodel.HSSFWorkbook;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;

public class CreateExcelCellFillColor {

 public static void main(String[] args) throws Exception {
  Workbook workbook = new XSSFWorkbook();
  //Workbook workbook = new HSSFWorkbook();

  CellStyle cellStyle = workbook.createCellStyle();
  cellStyle.setAlignment(HorizontalAlignment.CENTER);
  cellStyle.setVerticalAlignment(VerticalAlignment.CENTER);

  cellStyle.setFillForegroundColor(IndexedColors.GREY_25_PERCENT.getIndex());
  cellStyle.setFillPattern(FillPatternType.SOLID_FOREGROUND);

  Sheet sheet = workbook.createSheet();
  Row row = sheet.createRow(0);
  Cell cell = row.createCell(0);
  cell.setCellValue("cell value");
  cell.setCellStyle(cellStyle);

  row.setHeightInPoints(50);
  sheet.setColumnWidth(0, 50 * 256);

  FileOutputStream out = null;
  if (workbook instanceof HSSFWorkbook) {
   out = new FileOutputStream("CreateExcelCellFillColor.xls");
  } else if (workbook instanceof XSSFWorkbook) {
   out = new FileOutputStream("CreateExcelCellFillColor.xlsx");
  }
  workbook.write(out);
  out.close();
  workbook.close();
 }
}

Result: