Unexpected change in background of header of custome jcombobox in swing

I try to make a custom jcombobox for my menu sidebar
my jcombobox has JButton as items because I found it easy to insert and align icon and text.
But my problem is when I choose a item in dropdown, the color of the header changes unexpectedly to the default color of component.

public class MyComboBoxRenderer extends JLabel implements ListCellRenderer<Object> {
	private String _title;
	private JButton header;
	public MyComboBoxRenderer(String title) {
		_title = title;
	}

	@Override
	public Component getListCellRendererComponent(JList<?> list, Object value,
			int index, boolean isSelected, boolean hasFocus) {
		JButton item = new JButton();
		item.setOpaque(true);
		item.setBorderPainted(false);
		item.setFocusPainted(false);
		item.setBackground(isSelected ? Frame.LIGHTER_COLOR : Frame.LIGHT_COLOR ); 
		ImageIcon dot = new ImageIcon("image/icon/orange_dot.png");
		if (index == -1){
			item.setText(_title);
			header=item;
			return item; 
		}
		else{
			item.setIcon(dot);
			item.setIconTextGap(30);
			item.setText(value.toString());
			return item;
		}
		
	}
	
	public JButton getHeader(){
		return header;
	}
}

public static class MyComboBoxUI extends BasicComboBoxUI {
		final JButton button= new JButton(EXPAND_ARROW);
        protected void installDefaults() {
            super.installDefaults();
        }

        @Override
        protected JButton createArrowButton() {
            button.setContentAreaFilled(false);
            button.setBorder(null);
            return button;
        }

        @Override
        public void configureArrowButton() {
            super.configureArrowButton();
        }
        
        public JButton getArrowButton(){
			return button;
		}
	}

public class ComboBox extends JComboBox{
	public boolean isExpanded = false;
	private MyComboBoxRenderer renderer;
	public ComboBox(){
		super();
		this.setUI(new MyComboBoxUI());
	}
	public ComboBox(String[] list){
		super(list);
		renderer = new MyComboBoxRenderer("Lists Of Vocab");
		this.setUI(new MyComboBoxUI());
		this.setFont(Frame.I_FONT);
		this.setForeground(Frame.FONT_COLOR);
		this.setBackground(Frame.LIGHT_COLOR);
		this.setRenderer(renderer);
		MyComboBoxUI ui = (MyComboBoxUI) this.getUI();
		JButton arrowButton = ui.getArrowButton();
		arrowButton.addActionListener((ActionEvent e)->{
			isExpanded = !isExpanded;
			if(isExpanded == true){
				arrowButton.setIcon(COLLAPSE_ARROW);
			}
			else{
				arrowButton.setIcon(EXPAND_ARROW);
			}
		});
	}
	public MyComboBoxRenderer getRenderer(){
		return renderer;
	}

I add this combobox to the sideBar

    private void addCheckBoxToSideBar(){
    	ComboBox lists = new ComboBox(listNames);
    	lists.setAlignmentX(Component.LEFT_ALIGNMENT); // have to have
    	lists.addItemListener(new ItemListener(){
			@Override
            public void itemStateChanged(ItemEvent e) {
            if (e.getStateChange() == ItemEvent.SELECTED) {
            	CardLayout cl = (CardLayout)(mainPane.getLayout());
                cl.show(mainPane, (String)e.getItem());[enter image description here][1]
            }
        }
    });

    	sideBar.add(lists);
    }

This is first:

https://i.stack.imgur.com/ctVnT.png

But when I click to choose an item, it changes to default color:

https://i.stack.imgur.com/KkhhO.png

Those are what I tried but they didn't work:

• to set the background of the header in the itemStateChanged
• get the JTextfield of BasicComboBoxEditor of it

I wonder whether updating UI causes this problem but I don't understand much about it.

In your MyComboBoxUI, try overriding this method :

@Override
public void paintCurrentValueBackground(Graphics g,Rectangle bounds,boolean hasFocus)
{
 Color t = g.getColor();
 g.setColor(your_color);
 g.fillRect(bounds.x,bounds.y,bounds.width,bounds.height);
 g.setColor(t);
}