<mx:NumberValidator id="validator" source="{combo}" property="selectedIndex" minValue="0" invalid="changeComboBorder(event)" valid="changeComboBorder(event)" />private function changeComboBorder(event:ValidationResultEvent):void { var validator:Validator = event.target as Validator; var comboBox:ComboBox = validator.source as ComboBox; if (event.type == ValidationResultEvent.VALID) { comboBox.setStyle("borderColor", 0xB7BABC); } else { comboBox.setStyle("borderColor", 0xFF0000); }}<mx:NumberValidator id="comboValidator" required="true" minValue="0" invalid="changeComboBorder(event)" valid="changeComboBorder(event)" triggerEvent="focusOut" source="{trialCombo}" property="selectedIndex" />