Код:
<html>
<head>
<script type="text/javascript">
function changeText()
{
var x=document.forms[0].elements[0]
x.options[x.selectedIndex].style.color="red"
x.options[x.selectedIndex].style.backgroundColor="yellow"
x.options[x.selectedIndex].style.fontWeight="bold"
}
</script>
</head>
<body>
<form>
Select your favorite fruit:
<select id="mySelect">
<option>Apple</option>
<option>Orange</option>
<option>Pineapple</option>
<option>Banana</option>
</select>
<br /><br />
<input type="button" onclick="changeText()" value="Change text of selected fruit">
</form>
</body>
</html>