<form name="Sel">
<select name="ColorList" size="5" onChange="this.style.backgroundColor=Sel.ColorList.options[Sel.ColorList.selectedIndex].value;">
<option value="red">Красный
<option value="gray">Серый
<option value="blue">Синий
<option value="green">Зеленый
<option value="yellow">Желтый
</select>
</form>