APACHE POI 4.1 : Set cell background color from hex code

As the other answer tells, usage of setFillForegroundColor(XSSFColor color) instead of using indexed colors is necessary in XSSFCellStyle when it comes to customized colors. But usage of indexed colors from org.apache.poi.ss.usermodel.IndexedColors is possible in XSSF too. And this will be the most compatible way if using customized colors is not necessary.

But also creating the XSSFColor from java.awt.Color should be avoided. The constructor XSSFColor(java.awt.Color clr, IndexedColorMap map) is marked "TEST ONLY". And java.awt.Color will not be available in some circumstances.

So if the need is "set cell background color from hex code" and the hex code is in a String, then org.apache.commons.codec.binary.Hex can be used to get an byte[] array from that String. Apache commons codec is one of apache poi's dependencies already. Then constructor XSSFColor(byte[] rgb, IndexedColorMap colorMap) can be used. IndexedColorMap has no usage until now. So it can be set null. If IndexedColorMap gets any usage later, then the code has to be adjusted anyway.

Example:

import java.io.FileOutputStream;
import org.apache.poi.ss.usermodel.*;
import org.apache.poi.xssf.usermodel.*;

import org.apache.commons.codec.binary.Hex;

class CreateXSSFColor {

 public static void main(String[] args) throws Exception {

  try (Workbook workbook = new XSSFWorkbook(); 
       FileOutputStream fileout = new FileOutputStream("Excel.xlsx") ) {

   String rgbS = "FFF000";
   byte[] rgbB = Hex.decodeHex(rgbS); // get byte array from hex string
   XSSFColor color = new XSSFColor(rgbB, null); //IndexedColorMap has no usage until now. So it can be set null.

   XSSFCellStyle cellStyle = (XSSFCellStyle) workbook.createCellStyle();
   cellStyle.setFillForegroundColor(color);
   cellStyle.setFillPattern(FillPatternType.SOLID_FOREGROUND);

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

   workbook.write(fileout);
  }

 }
}