Colorize Icons Filter Java

[Virux]

Okay.
I have been seriously struggling with this lately. I've searched and searched forever. The closest I found was this thread here: http://72.5.124.102/thread.jspa?messageID=1662301

What I am trying to acheive:
I want to give my interface grayscale images for icons(png), and have a filter class that can color the grayscale images to a certain color, without changing the black/gray/white values. The effect I am looking for is exactly like the Color blending mode on Photoshop, Gimp has it too.

Even though the color is changing, the actual image should not. Similar to a grayscale effect, but instead of gray it uses a color.
So the image shouldn't be getting darker, lighter, distorted, etc. Just a colored tint. The effect I'm talking about is far different than just putting a semi-transparent panel over it with the color I want.

I have found this exact type of effect in a Java image editor from JHLabs, and I've attempted to demystify the code of the application but I cannot seem to find out how they are doing it.

Here is my current code, taken from the URL outlined at the top of this post. I've also got a demo interface, that paints "TheIcon.png", waits one second, filters the image and repaints the same image.
The problem is, after it filters all I get is the black interface. I don't know if the image is there, or if it's just completely transparent. But it's not working D=

I would like the chance to understand what this code is doing, but I can't find any articles or anything like that-- at least none that I can understand. I've been reluctant to read them because the first few I went through were doing something completely different. Anyways here's the code. =)

Interface.java

import java.awt.*;
import java.net.URL;

import javax.swing.*;

public class Interface extends JFrame {
	private static final long serialVersionUID = 1L;
	
	private static JPanel contentPane;
	private static Image displayedImage;


	public Interface() { 
		super();
		setImage();
		initializeComponent(); this.setVisible(true);
		try { Thread.sleep(1000); } catch(Exception e) {  }
		filterImage();
	}

	private void initializeComponent() {
		
		contentPane = new JPanel() {
			private static final long serialVersionUID = 1L;

			protected void paintComponent(Graphics g) {
				Dimension d = getSize();
				g.setColor(new Color(0,0,0));
				g.fillRect(0, 0, d.width, d.height);
				g.drawImage(displayedImage, 0, 0, d.width, d.height, null);
			}
		};
		
		this.setContentPane(contentPane);
		this.setTitle("Colorize Test");
		this.setSize(new Dimension(340, 250));
		this.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
		this.setResizable(false);
	}
	
	public static void filterImage() {
		displayedImage = ColorFilter.filterImage(displayedImage);
		contentPane.repaint();
	}
	
	private void setImage() {
		// Set image
		URL imagePath = getClass().getResource("TheIcon.png");
		ImageIcon tmp = null;
		try { 
			tmp = new ImageIcon(imagePath);
		} catch(Exception e) { 
			System.out.println("Could not find the image in the path given");
			System.exit(1);
		}
		displayedImage = tmp.getImage();
	}
	
	public static void main(String[] args) {
		try {
			UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
		} catch (Exception e) { }
		new Interface();
	}
	
}

ColorFilter.java

import java.awt.Color;
import java.awt.Image;
import java.awt.Toolkit;
import java.awt.image.FilteredImageSource;
import java.awt.image.ImageFilter;
import java.awt.image.RGBImageFilter;


public class ColorFilter extends RGBImageFilter {
	private float hue;
	
	public static Image filterImage(Image image) {
		ImageFilter filter = new ColorFilter(Color.red);
		FilteredImageSource filteredSrc = new FilteredImageSource(image.getSource(), filter);
		image = Toolkit.getDefaultToolkit().createImage(filteredSrc);
		
		return image;
	}
	
	public ColorFilter(Color c) {
		  float[] hsb = Color.RGBtoHSB(c.getRed(), c.getGreen(), c.getBlue(), null);
		  hue = hsb[0];
		  canFilterIndexColorModel = true;
	}
		 
	public int filterRGB(int x, int y, int rgb) {
		//detect the transparency of the pixel
		int alpha = rgb & 0xff000000;
		//if the pixel is fully transparent make nothing
		if (alpha == 0) { return rgb; }
		float[] hsb = Color.RGBtoHSB((rgb >> 16) & 0xff, (rgb >> 8) & 0xff, rgb & 0xff, null);
		//change the pixel to the new color
		hsb[0] = hue;
		//add the transparency of the pixel and return it
		return (Color.HSBtoRGB(hsb[0], hsb[1], hsb[2]) & 0xffffff) | alpha;
	}

}