FULL PRODUCT VERSION :
java version  " 1.7.0_13 " 
Java(TM) SE Runtime Environment (build 1.7.0_13-b20)
Java HotSpot(TM) Client VM (build 23.7-b01, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
Ubuntu 12.04.1 LTS

A DESCRIPTION OF THE PROBLEM :
Using the  " GradientTranslucentWindowDemo "  example from http://docs.oracle.com/javase/tutorial/uiswing/misc/trans_shaped_windows.html you can see the result of window translucency working.  However, if you gradually decrease any of the color components (R, G, or B), the color compositing is incorrect.

isWindowTranslucencySupported(PERPIXEL_TRANSLUCENT) report true.

Testing Windows 7, Vista and Mac OS X Mountain Lion; all work as expected.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Get the  " GradientTranslucentWindowDemo "  example from http://docs.oracle.com/javase/tutorial/uiswing/misc/trans_shaped_windows.html.

Change any of the color components of the gradient (final int R,G, or B) to something lower, i recommend 0 to see the full effect.

Run the app.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
You should see the color defined by R, G, and B with a gradient alpha value, starting at full transparent, and ending fully opaque.

On windows 7:
http://imageshack.us/photo/my-images/834/windows7s.png/
ACTUAL -
The background of the frame is the gradient color specified but is mixed with the desktop/background windows incorrectly.

On Ubuntu:
http://imageshack.us/photo/my-images/805/ubuntuw.png/

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import static java.awt.GraphicsDevice.WindowTranslucency.PERPIXEL_TRANSLUCENT;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GraphicsDevice;
import java.awt.GraphicsEnvironment;
import java.awt.GridBagLayout;
import java.awt.Paint;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.UIManager;
import javax.swing.UIManager.LookAndFeelInfo;

public class GradientTranslucentWindowDemo extends JFrame {
    public GradientTranslucentWindowDemo() {
        super( " GradientTranslucentWindow " );

        setUndecorated(true);
        setBackground(new Color(0,0,0,0));
        setSize(new Dimension(300,200));
        setLocationRelativeTo(null);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel panel = new JPanel() {
            @Override
            protected void paintComponent(Graphics g) {
                if (g instanceof Graphics2D) {
                    final int R = 0;  // HERE IS THE ONLY CHANGE
                    final int G = 240;
                    final int B = 240;

                    Paint p =
                        new GradientPaint(0.0f, 0.0f, new Color(R, G, B, 0),
                            0.0f, getHeight(), new Color(R, G, B, 255), true);
                    Graphics2D g2d = (Graphics2D)g;
                    g2d.setPaint(p);
                    g2d.fillRect(0, 0, getWidth(), getHeight());
                }
            }
        };
        setContentPane(panel);
        setLayout(new GridBagLayout());
        add(new JButton( " I am a Button " ));
        
        setAlwaysOnTop(true);
    }

    public static void main(String[] args) {
        // Determine what the GraphicsDevice can support.
        GraphicsEnvironment ge =
            GraphicsEnvironment.getLocalGraphicsEnvironment();
        GraphicsDevice gd = ge.getDefaultScreenDevice();
        boolean isPerPixelTranslucencySupported =
            gd.isWindowTranslucencySupported(PERPIXEL_TRANSLUCENT);

        //If translucent windows aren't supported, exit.
        if (!isPerPixelTranslucencySupported) {
            System.out.println(
                 " Per-pixel translucency is not supported " );
                System.exit(0);
        }

        try {
            for (LookAndFeelInfo info : UIManager.getInstalledLookAndFeels()) {
                if ( " Nimbus " .equals(info.getName())) {
                    UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (Exception e) {
            // If Nimbus is not available, you can set the GUI to another look and feel.
        }
        
        //JFrame.setDefaultLookAndFeelDecorated(true);

        // Create the GUI on the event-dispatching thread
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                GradientTranslucentWindowDemo gtw = new GradientTranslucentWindowDemo();

                // Display the window.
                gtw.setVisible(true);
            }
        });
    }
}

---------- END SOURCE ----------