customizer_scripts.js

( function( $ ) {
    $( window ).load( function() {

        // When 'Use Portal Color Controls' customizer control is toggled
        wp.customize( 'portal_color_control', function ( value ) {
            value.bind( function( newval ) {

                // If toggled off, warn user that portal color settings will be deleted
                if ( 'off' == newval ) {
                    var confirmation = window.confirm( "WARNING: turning this off will delete all the portal color settings. Continue?" );

                    if ( true == confirmation ) {
                        // If user confirms, run code here to delete all portal color settings
                    } else {
                        // If user does not confirm, reset the customizer control to 'on'
                        this.set( 'on' );
                    }

                }
            });
        });

    });
})( jQuery );