different colors for nav buttons using CSS and PHP

Below is the code I am using which creates navigation menu dynamically.

I am trying to edit the code so that each menu button has a different colour.

I can see where has_nav_menu comes in to create the necessary standards in the array (‘menu_id’ gets set to ‘dropmenu’). How can I set it to different divs eg ‘dropmenu-art’ which are already set up in CSS. Alternatively I could just change background color in If loop below?

Please help!

<?php
	//NORMAL MENU
	if (has_nav_menu( 'main' ) ) { wp_nav_menu(array('theme_location' => 'main', 'container_id' => 'navigation', 'menu_id' => 'dropmenu')); }
	//SELECT MENU
	$menu_name = 'main';

    if ( ( $locations = get_nav_menu_locations() ) && isset( $locations[ $menu_name ] ) ) {
		$menu = wp_get_nav_menu_object( $locations[ $menu_name ] );
		$menu_items = wp_get_nav_menu_items($menu->term_id);
		$menu_list = '<select id="selectMenu"><option value="" selected="selected">'.__('Menu','themolitor').'</option>';

		foreach ( (array) $menu_items as $key => $menu_item ) {
	    	$title = $menu_item->title;
	   		$url = $menu_item->url;
	   		$menu_list .= '<option value="' . $url . '">' . $title . '</option>';

			/* -- set menu colours -- */
			if ($title == "Art") {
					/* -------- How do I change "menu_id' from 'dropmenu' to 'dropmenu-art'  ????? ----------*/
				} else if ($title == "Events") {
					/*   perhaps there is a method to just change CSS "background: #000;" property here instead?   */
				} else if ($title == "Influences") {
					/*   etc   */
				} else if ($title == "Links") {
					/**/
				} else if ($title == "Personal") {
					/**/
				} else if ($title == "Portraits") {
					/**/
				} else if ($title == "Press") {
					/**/
				} else if ($title == "Software") {
					/**/
				} else if ($title == "Technical") {
					/**/
				} else if ($title == "Video") {
					/**/
				} else {
					/**/
			}
		}
		$menu_list .= '</select>';
		echo $menu_list;
    }
    ?>