oppure, risparmiando ancora codice:
Codice PHP:
<?php
if($value == $N1 or $value == $N2 or $value == $N3 or $value == $N4 ) $color = "red";
else $color = "black";
?>
<input type='text' id='S1' tabIndex='1' value='Paolo' name='S1' style='background-color:<?php echo $color; ?>' />