<?php 
$k =1;
for($v = 0; $v < $nt; $v++) {         
	$bgClass= 'blueBg';
		if($k > 1 && $k%2 == 0){
			$bgClass= 'whiteBg';
		}
?> 
<tr  class="row_tr <?php echo $bgClass;?>">						
			<td class="row_td td_column_0">&nbsp;</td>					
			<td class="row_td column_1"><?php echo $rows[$v]['nothing'];?></td>
			...
			<td class="row_td td_column_0">&nbsp;</td>
	</tr>

<?php       
	$k++;
	}	
?>