如何在Magento中以编程方式设置产品属性

我有一个Magento实例，我使用以下代码：

include_once '../app/Mage.php';
Mage::app();
try{
     $product_id = '';
     // get query string
     if (!isset($_GET['product_id'])) { $product_id = ''; } else { $product_id = $_GET['product_id']; }
     if (!isset($_GET['qty'])) { $qty = '1'; } else { $qty = $_GET['qty']; }
     $product = Mage::getModel('catalog/product')->load($product_id);
     $session = Mage::getSingleton('core/session', array('name'=>'frontend'));
     $cart = Mage::helper('checkout/cart')->getCart();
     $cart->addProduct($product, $qty);
     $session->setLastAddedProductId($product->getId());
     $session->setCartWasUpdated(true);
     $cart->save();
     $result = "{'result':'success'}";
     echo $result;
} catch (Exception $e) {
     $result = "{'result':'error'";
     $result .= ", 'message': '".$e->getMessage()."'}";
     echo $result;
}

但是，我收到一个错误，上面写着"请指定产品所需的选项"如何在上一个代码中设置一个名为"Colors"并默认为"black"的属性？谢谢