import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;

public class Farbwechsel extends JFrame implements ActionListener {

    JButton bRot;
    JButton bBlau;

    public Farbwechsel() {
        setSize(200, 100);
        setTitle("Farbwechsel");
        getContentPane().setBackground(Color.ORANGE);
        bRot = new JButton("Rot");
        bBlau = new JButton("Blau");
        getContentPane().setLayout(new FlowLayout());
        getContentPane().add(bRot);
        getContentPane().add(bBlau);
        bRot.addActionListener(this);
        bBlau.addActionListener(this);
        bRot.setActionCommand("rot");
        bBlau.setActionCommand("blau");
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setVisible(true);
    }

    public void actionPerformed(ActionEvent e) {
        if (e.getActionCommand().equals("rot")) {
            getContentPane().setBackground(Color.red);
        } else {
            getContentPane().setBackground(Color.blue);
        }
        repaint();
    }
}

public class Test {
    public static void main(String[] args) {
        Farbwechsel demo = new Farbwechsel();
    }
}