[hardwired]

With a continuously–running loop that is busy styling all of the text in the textPane, called by a DocumentListener, I would say that your app could run very slow.
A lighter–weight way to do this is with a simple DocumentFilter. Here's one way you could put it together:

Code:

//  <applet code="TEV3Rx" width="400" height="400"></applet>
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.text.*;

public class TEV3Rx extends JApplet {
    public void init() {
        JTextPane textPane = new JTextPane();
        Style style = textPane.addStyle("basic", null);
        StyleConstants.setFontSize(style, 18);
        StyleConstants.setForeground(style, Color.green.darker());
        StyleConstants.setFontFamily(style, "Lucida Sans Unicode");
        textPane.setLogicalStyle(style);
        style = textPane.addStyle("redDigits", null);
        StyleConstants.setForeground(style, Color.red);
        AbstractDocument doc = (AbstractDocument)textPane.getDocument();
        doc.setDocumentFilter(new DigitFilter(style));
        getContentPane().add(textPane);
    }

    public static void main(String[] args) {
        JApplet applet = new TEV3Rx();
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setContentPane(applet);
        f.setSize(400,400);
        f.setLocation(200,200);
        applet.init();
        f.setVisible(true);
    }
}

class DigitFilter extends DocumentFilter {
    String domain = "0123456789";
    Style digitStyle;

    public DigitFilter(Style style) {
        digitStyle = style;
    }
 
    public void insertString(DocumentFilter.FilterBypass fb,
                             int offset,
                             String str,
                             AttributeSet attrs) throws BadLocationException {
        replace(fb, offset, 0, str, attrs);
    }
 
    public void replace(DocumentFilter.FilterBypass fb,
                        int offset,
                        int length,               // selected text
                        String str,               // incoming string
                        AttributeSet attrs) throws BadLocationException {
        char[] source = str.toCharArray();
        char[] result = new char[source.length + 2];
        int k = 0;
        for(int j = 0; j < source.length; j++) {
            result[k++] = source[j];
            fb.replace(offset, length, new String(result, 0, k), attrs);
            if(domain.indexOf(source[j]) != -1) {
                StyledDocument doc = (StyledDocument)fb.getDocument();
                doc.setCharacterAttributes(offset, 1, digitStyle, false);
            }
        }
    }
}