Magento – Get value of selected multiselect attribute

Since you want to do this in the action catalog_product_save_after you could simple update the individual product attribute and not the complete product, thus stopping the infinite loop problem.

$attrCode = 'your_attribute';
$sourceModel = Mage::getModel('catalog/product')->getResource()
    ->getAttribute($attrCode)->getSource();
$valuesText = explode(',', 'red,green,blue');
$valuesIds = array_map(array($sourceModel, 'getOptionId'), $valuesText);
$product->setData($attrCode, $valuesIds);
$product->getResource()->saveAttribute($product, $attrCode);

After a lot of digging I found a solution why my attribute values wasn't able to be assigned to product. In "if" add:

$data['backend'] = 'Magento\Eav\Model\Entity\Attribute\Backend\ArrayBackend';

Whole if:

if (!$multiSelectData === false && is_array($multiSelectData)) {
        $data['input'] = 'multiselect';
        $data['option'] = array('values' => $multiSelectData);
        $data['backend'] = 'Magento\Eav\Model\Entity\Attribute\Backend\ArrayBackend';
}