Prova questo

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<style type="text/css">
<!--
.pippo {background-color:red}
-->
</style>
<script language="JavaScript" type="text/javascript">
<!--
function resetColor(f){
	for (var i=0; i<f.elements.length; i++) {
	  curObj = f.elements[i];
		document.getElementById(curObj.id).style.backgroundColor="";
	}
}
//-->
</script>

</head>
<body>
<form id="myForm" name="myForm" >
 <input type="text" id="t1" style="background-color:red">
 <input type="text" id="t2" style="background-color:red">
 <input type="text" id="t3" style="background-color:red">
 <input type="text" id="t4" style="background-color:red">   
</form> 
<input type="button" value="Ripulisci" onclick="resetColor(document.getElementById('myForm'))">
</body>
</html>