fai in questo modo:
codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Documento senza titolo</title>
<script language="javascript">
function tabella_bicolore(){
	//1. Raggruppiamo tutti i tag <tr>
	var righe= document.getElementsByTagName("tr")
	alert("Entra")
	for(i=0;i<righe.length;i++){
	if((i+1)%2==0){

righe[i].style.backgroundColor="#009";}
else{righe[i].style.backgroundColor="white";}
}

}
</script>

</head>

<body>
<table border="1" width="100%">
<tr><td>Ciao</td></tr>
<tr><td>Ciao</td></tr>
<tr><td>Ciao</td></tr>
<tr><td>Ciao</td></tr>
<tr><td>Ciao</td></tr>
<tr><td>Ciao</td></tr>
<tr><td>Ciao</td></tr>

</table>


<input type="button" onclick="tabella_bicolore();" Value="Cambia" />
</body>
</html>