Q:

How do I change color of a particular word document using apache poi?

Current output:

Needed output:

above are the snapshots of the .docx and below is the code sample code, I want to change the color of a after it is replaced by @. r.setColor("DC143C") doesn't work:
for (XWPFParagraph p : docx.getParagraphs()) {
        List<XWPFRun> runs = p.getRuns();
        if (runs != null) {
            for (XWPFRun r : runs) {
                String origText = r.getText(0);
                if (origText != null && origText.contains("a")) {
                    origText = origText.replace("a", "@");
                    r.setText(origText, 0);
                }
            }
        }
    }

A:

If the need is to change the color of just one character then this character must be in its own run. This is because only runs can be styled.
If you have a document containing text already then you must run through all already existing runs and possible split those runs into multiple ones. As the result each string part which shall be styled separately must be in its own run, also if it is only one character.
Example:
import java.io.*;
import org.apache.poi.xwpf.usermodel.*;

import java.awt.Desktop;

import org.apache.poi.openxml4j.exceptions.InvalidFormatException;

public class WordReadAndWrite {

 public static void main(String[] args) throws IOException, InvalidFormatException {

  XWPFDocument doc = new XWPFDocument(new FileInputStream("source.docx"));

  for (XWPFParagraph p : doc.getParagraphs()) { //go through all paragraphs
   int runNumber = 0;
   while (runNumber < p.getRuns().size()) { //go through all runs, we cannot use for each since we will possibly insert new runs
    XWPFRun r = p.getRuns().get(runNumber);
    String runText = r.getText(0);
    if (runText != null && runText.contains("a")) { //if we have a run with an "a" in it, then
     char[] runChars = runText.toCharArray();
     StringBuffer sb = new StringBuffer();
     for (int charNumber = 0; charNumber < runChars.length; charNumber++) { //go through all characters in that run     
      if (runChars[charNumber] == 'a') { //if the charcter is an 'a' then      
       r.setText(sb.toString(), 0); //set all characters, which are current buffered, as the text of the actual run
       r = p.insertNewRun(++runNumber); //insert new run for the '@' as the replacement for the 'a'
       r.setText("@", 0);
       r.setColor("DC143C");
       r = p.insertNewRun(++runNumber); //insert new run for the next characters
       sb = new StringBuffer(); //empty buffer
      } else {
       sb.append(runChars[charNumber]); //buffer all characters which are not 'a's
      }
     }
     r.setText(sb.toString(), 0); //set all characters, which are current buffered, as the text of the actual run
    }
    runNumber++;
   }
  }

  doc.write(new FileOutputStream("result.docx"));
  doc.close();

  System.out.println("Done");
  Desktop.getDesktop().open(new File("result.docx"));

 }
}

