Background color of the selected item in an uneditable JComboBox

This should work

jComboBox1.setRenderer(new DefaultListCellRenderer() {
    @Override
    public void paint(Graphics g) {
        setBackground(Color.WHITE);
        setForeground(Color.BLACK);
        super.paint(g);
    }
});

The background assigned by the renderer is overriden by the selection background color of the JList that is used in the popup for the combo box. Check out the "paintCurrentValue" method of the BasicComboBoxUI class. So the workaround would be:

JComboBox comboBox = new JComboBox(...);
Object child = comboBox.getAccessibleContext().getAccessibleChild(0);
BasicComboPopup popup = (BasicComboPopup)child;
JList list = popup.getList();
list.setSelectionBackground(Color.RED);

This will affect the rendering of the popup as well. If you don't want it to affect the popup then you will need to create a custom renderer to specifically set the background of selected items.