If you want to get simple product's custom attribute value of a configurable product, like Color is your custom dropdown attribute of simple product and if you want to display available color of a configurable product on your category OR product list page or product details page, you can try following solution.

<?php 
  $simpleCollection = "";
  if($_product->getTypeId() == "configurable"){
     $conf = Mage::getModel('catalog/product_type_configurable')->setProduct($_product);
     $simpleCollection = $conf->getUsedProductCollection()->addAttributeToSelect('*');
     
     foreach($simpleCollection as $simpleProduct){
            $optLbl = $simpleProduct->getAttributeText('color');
            echo $simpleProduct->getId()." ".$simpleProduct->getSku()." ".$optLbl."<br>";
     }
   }
?>