Setting color of specific menu items depending on page or post tag

OK, got it 🙂

add_filter( 'wp_nav_menu_objects', 'add_nav_menu_item_class' );

function add_nav_menu_item_class( $items ) {
    foreach ( $items as $item ) {
        $post_id = get_post_meta( $item->ID, '_menu_item_object_id', true );
        $post_tags = get_the_tags( $post_id );
        if ( $post_tags ) {
            foreach( $post_tags as $tag ) {
                if  ( $tag->name == 'unwatched' ) {
                    $item->classes[] = 'unwatched';
                }
            }
        }
    }
    return $items;
}

The associated CSS, which changes the color of all menu items tagged with the ‘unwatched’ class, is

li.unwatched > a {
    color: FORESTGREEN;
}