Už jsem to udělal po svém.

...
java.awt.Font font = new java.awt.Font("Times New Roman", 2, 20);
Color color_list[] = {Color.RED, Color.GREEN, Color.YELLOW, Color.BLUE};
this.getContentPane().add(color_player_1);
this.getContentPane().add(color_player_2);
this.getContentPane().add(color_focus);
color_player_1.setModel(new javax.swing.DefaultComboBoxModel<>(new String[]{"RED", "GREEN", "YELLOW", "BLUE"}));
color_player_1.setFont(font);
color_player_1.setSelectedIndex(0);
color_player_1.setBounds(5, 30, 110, 30);
color_player_1.setBackground(Color.RED);
color_player_2.setModel(new javax.swing.DefaultComboBoxModel<>(new String[]{"RED", "GREEN", "YELLOW", "BLUE"}));
color_player_2.setFont(font);
color_player_2.setSelectedIndex(0);
color_player_2.setBounds(120, 30, 110, 30);
color_player_2.setBackground(Color.RED);
color_focus.setModel(new javax.swing.DefaultComboBoxModel<>(new String[]{"RED", "GREEN", "YELLOW", "BLUE"}));
color_focus.setFont(font);
color_focus.setSelectedIndex(0);
color_focus.setBounds(235, 30, 110, 30);
color_focus.setBackground(Color.RED);
ItemListener item_listener = (ItemEvent e) -> {
((JComboBox) e.getSource()).setBackground(color_list[((JComboBo x) e.getSource()).getSelectedIndex()]);
};
color_player_1.addItemListener(item_listener);
color_player_2.addItemListener(item_listener);
color_focus.addItemListener(item_listener);