A DESCRIPTION OF THE REGRESSION :
setBackground(java.awt.Color), not work in swing components but yes work in awt components

REPRODUCIBLE TESTCASE OR STEPS TO REPRODUCE:
import java.awt.*;
import javax.swing.*;

public miJFrame extends JFrame {

public miFrame() {
setSize(300, 300);
setBackground(Color.BLACK);
}

public static void main(String[] args) {
new miFrame().setVisible(true);
}

}

RELEASE LAST WORKED:
5.0

RELEASE TEST FAILS:
mustang-beta

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The swing component's background shoud be the color specified
ACTUAL -
The swing component's background don't show the color