<script>
function checkContactForm (strFile) {
if (document.contactform1.colore.selectedIndex==0) {
alert("devi selezionare un colore")
} else {
document.contactform1.submit()
}
}
</script>
</head>

<body>
<form name="contactform1" method="post" action="index1.htm">
...
"><input type="button" name="Submit" value="Invia" onclick="checkContactForm()">
...

ciao