customize of label "choose an option" of configurable product in magento

Step 1: Change in catalog/product/view/type/options/configurable.phml

 <?php
$_product = $this->getProduct();
$_attributes = Mage::helper('core')->decorateArray($this->getAllowAttributes());
?>
<?php if ($_product->isSaleable() && count($_attributes)):?>
    <dl>
    <?php foreach($_attributes as $_attribute): ?>
        <?php
        $_attributeId = $_attribute->getAttributeId();
        $_attributeInfo = Mage::getModel('eav/entity_attribute')->load($_attributeId);
        $_attributeLabel = str_replace(' ','-',strtolower($_attributeInfo->getFrontendLabel()));
        ?>
            <dd<?php if ($_attribute->decoratedIsLast){?> class="last"<?php }?>>
                <div class="input-box">
                    <select name="super_attribute[<?php echo $_attribute->getAttributeId() ?>]" id="attribute<?php echo $_attribute->getAttributeId() ?>" class="required-entry super-attribute-select kevin-black-<?php echo $_attributeLabel;?>">
                    <option><?php echo 'choose a '.$_attributeInfo->getFrontendLabel() ?></option>
                    </select>
                </div>
            </dd>
        <?php endforeach; ?>
    </dl>
<script type="text/javascript">
    var spConfig = new Product.Config(<?php echo $this->getJsonConfig() ?>);
    //kevin.qazware@gmail.com Change Text follow attribute Label
    function changeFristText(){
        <?php foreach($_attributes as $_attribute): ?>
            <?php
            $_attributeId = $_attribute->getAttributeId();
            $_attributeInfo = Mage::getModel('eav/entity_attribute')->load($_attributeId);
            ?>
            var label = '<?php echo $_attributeInfo->getFrontendLabel();?>';
            $$('select.kevin-black-'+label).each(function(elem){
                var options = elem.childElements();
                options[0].update(label);
            });
        <?php endforeach;?>
    }
</script>
<?php endif;?>

Step 2: js/varien/configurable.js

replace line 171 = element.options[0] = new Option(element.config.label, '');

Step 3: app/design/frontend/default/grayscale/template/catalog/product/view/type/options/configurable.php
change this line:

'label' => 'choose a '.$attribute->getLabel(),