update cart using color and size attributes in magento

How do i update cart using 'color' and 'size' attributes along with 'qty'.

here i am able to update using qty.

here i am passing API url in the following way:

http://<host>/middleware/update_cart.php?cart_id=5&product_list=51@9

$arr = explode (',',$_request['product_list']) ;

        $productArr = array() ;
        foreach ($arr as $value)
        {
            $contentArr = explode ('@',$value) ;
            $temp = array() ;
            $temp['product_id']=$contentArr[0] ;
            $temp['qty']=$contentArr[1] ;
            array_push($productArr,$temp) ;
        }

        $finalArr = array() ;
        $finalArr[0] = $cart_id ;
        $finalArr[1] = $productArr ;
 $status = $client->call( $session,'hmcheckout_product.update',$finalArr);

Qty is inventory provided to product

Color and Size are something that are chosen from available set of options on user end. For that you have to create configurable products

For creating configurable product follow this http://www.magentocommerce.com/knowledge-base/entry/tutorial-creating-a-configurable-product

Hope now you understand what is required