Set dropdown value using text value

This is untested but should work:

$colourId = $product->getResource()->getAttribute("colour")->getSource()->getOptionId("red");
$product->setColour($colourId)->save();

You can do it by using below mentioned code:

<?php
$attrId = attributeValueExists('color', 'red');
$product->setData('color', $attrId);
$product->save();

function attributeValueExists($argAttribute, $argValue)
{
    $attributeModel        = Mage::getModel('eav/entity_attribute');
    $attributeOptionsModel = Mage::getModel('eav/entity_attribute_source_table') ;

    $attributeCode         = $attributeModel->getIdByCode('catalog_product', $argAttribute);
    $attribute             = $attributeModel->load($attributeCode);

    $options               = $attributeOptionsModel->getAllOptions(false);

    foreach ($options as $option) {
        if ($option['label'] == $argValue) {
            return $option['value'];
        }
    }

    return false;
}

Just use attributeValueExists('attribute_code','attribute text value') this function in your code.Hope it will help you.