Code:
/

package javaapplication16;
import java.awt.Graphics.*;
import java.awt.*;
 
public class Test extends javax.swing.JFrame {

   
    public Test() {
        initComponents();
    }

   
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">//GEN-BEGIN:initComponents
    private void initComponents() {

        slika = new javax.swing.JLabel();
        zeleni = new javax.swing.JButton();
        crveni = new javax.swing.JButton();
        crtaj = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        slika.setText(" ");

        zeleni.setText("zeleni");
        zeleni.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                zeleniActionPerformed(evt);
            }
        });

        crveni.setText("crveni");
        crveni.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                crveniActionPerformed(evt);
            }
        });

        crtaj.setText("crtaj");
        crtaj.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                crtajActionPerformed(evt);
            }
        });

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGap(34, 34, 34)
                .addComponent(zeleni)
                .addGap(18, 18, 18)
                .addComponent(crveni)
                .addGap(18, 18, 18)
                .addComponent(crtaj)
                .addContainerGap(155, Short.MAX_VALUE))
            .addComponent(slika, javax.swing.GroupLayout.DEFAULT_SIZE, 400, Short.MAX_VALUE)
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                .addComponent(slika, javax.swing.GroupLayout.PREFERRED_SIZE, 222, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 44, Short.MAX_VALUE)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(zeleni)
                    .addComponent(crveni)
                    .addComponent(crtaj))
                .addContainerGap())
        );

        pack();
    }// </editor-fold>//GEN-END:initComponents

    private void zeleniActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_zeleniActionPerformed
       slika.setIcon(new javax.swing.ImageIcon(getClass().getResource("/zeleni.gif")));
        Graphics g= slika.getGraphics();
         g.setColor(Color.WHITE);
         g.drawLine(50,100,100,100);
}//GEN-LAST:event_zeleniActionPerformed

    private void crveniActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_crveniActionPerformed
        slika.setIcon(new javax.swing.ImageIcon(getClass().getResource("/crveni.gif")));
        Graphics g= slika.getGraphics();
         g.setColor(Color.WHITE);
         g.drawLine(50,100,100,100);
    }//GEN-LAST:event_crveniActionPerformed

    private void crtajActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_crtajActionPerformed
        Graphics g= slika.getGraphics();
         g.setColor(Color.WHITE);
         g.drawLine(50,100,100,100);
    }//GEN-LAST:event_crtajActionPerformed

   
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new Test().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify//GEN-BEGIN:variables
    private javax.swing.JButton crtaj;
    private javax.swing.JButton crveni;
    private javax.swing.JLabel slika;
    private javax.swing.JButton zeleni;
    // End of variables declaration//GEN-END:variables

}