<div class="product-categories">
   <b> Stok Durumu:
  <?php 
    $color = "green";
     if( $urun['stok'] == 0){
         $color = "red";
     }
  ?>
    <span class="product-category" style="color: <?php echo $color; ?>;"><?php echo $urun['stok']; ?> Adet</span></b>
</div>