SO jQuery Fiddle
Test Fiddle mainly for SO Questions jQuery 1.4.3, UI 1.8.5
by Nick Craver
HTML
<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.5/themes/ui-lightness/jquery-ui.css">
<select id="color"><option value="">Select Color</option>
<option value="1">Red</option>
<option value="2">Blue</option>
<option value="3">Black</option>
<option value="4">Purple</option>
</select>
JavaScript
$('#color').val(3);