add_filter( 'woocommerce_add_cart_item_data', 'filter_add_cart_data', 10, 3
);
function filter_add_cart_data( $cart_item_data, $product_id){

    if (isset($_POST['dropdown_color']))
    {
        $color =  esc_attr($_POST['dropdown_color']);
    }

    $cart_item_data['custom'] = array(
        'color'  => $color,
        'unique_key' => md5(microtime().rand())
    );

    return $cart_item_data;
}