Un aiutino:
<html>
<head>
<script type="text/javascript">
function visualizza(obj, a){
var e=document.getElementById(a);
if(!e)return true;
e.style.display="block"
return true;
}

function nascondi(obj, a){
var e=document.getElementById(a);
if(!e)return true;
e.style.display="none"
return true;
}

</script>
<title></title>
</head>
<body>
<form name="Form">

<table border=0 width="100%" cellspacing="3" cellpadding="2">

<tr>
<td><div align="right">Effettua una scelta<span class="Stile2">.......:</span></div></td>
<td>
<select size="1" name="pippo" onchange="return visualizza(this, 'nascosto')">
<option value="" onselect="return nascondi(this, 'nascosto')" selected>Seleziona</option>
<option value="Nessun colore" onselect="return visualizza(this, 'nascosto')">Nessun colore</option>
<option value="rosso" onselect="return visualizza(this, 'nascosto')">rosso</option>
<option value="verde" onselect="return visualizza(this, 'nascosto')">verde</option>
<option value="giallo" onselect="return visualizza(this, 'nascosto')">giallo</option>
</select></td>
</tr> </table>
<div id="nascosto" style="display: none; margin-left: 20px;">
<table><tr>
<td>Se hai scelto un colore, motiva la ragione...</div></td>
<td>
<input type="text" name="" size="33"></td>
</tr>
</table></div>
</form>
</body>
</html>