From the original submitter:

FULL PRODUCT VERSION :
java version "1.6.0-beta"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.6.0-beta-b59)
Java HotSpot(TM) Client VM (build 1.6.0-beta-b59, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
The WindowsXP look&feel uses a wrong background color for JButtons in a toolbar when the silver theme is displayed. The background color for JButtons is different from JToggleButtons and JToolbars, which looks quite awkward in some cases. This problem does not occur with the Blue (default) and Olive Green themes.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
  To see the correct behaviour set the theme on Windows XP to Blue or Olive Green, and start the test case. The four buttons at the top right (next to the combo box) all have the same background.

Close the file dialog and change the Windows XP theme to Silver. Start the application again and you will see that the first two of the four buttons have a different (darker) background color. These two buttons are JButtons, the other two are JToggleButtons.


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
JButtons in a toolbar should have the same background color as JToggleButtons and JToolbars.
ACTUAL -
The background color of JButtons is different from JToggleButtons and JToolbars when the Silver theme is used.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import javax.swing.*;

public class WinXPButtonTest {

  public static void main(String[] args) {
    try {
      UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
    } catch (Exception ex) {
      ex.printStackTrace();
    }
    JFileChooser jFileChooser = new JFileChooser();
    jFileChooser.showOpenDialog(null);
    System.exit(0);
  }

}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Unknown