[bes]

не вникал, что там должно меняться, для примера меняю фон таблицы и фон первой ячейки этой таблицы

<form>
<select>
	<option>red</option>
	<option>green</option>
	<option>blue</option>
	<option>yellow</option>
</select> 
<select>
	<option>red</option>
	<option>green</option>
	<option>blue</option>
	<option>yellow</option>
</select> 

<table style="background: yellow; border: solid 1px">
	<tr>
		<td style="background: red">ааа
		<td style="background: yellow">bbb 
	</tr>
	<tr>
		<td style="background: green">ccc 
		<td style="background: blue">ddd 
	</tr>
</table>
</form>


<script>
window.onload = function () {
	document.forms[0].onchange = function (e) {
		e = e || event;
		var target = e.target || e.srcElement;
		if (target.tagName == 'SELECT') {
			if (target == this.children[0]) {
				var elem = this.children[2];
			} else 
			if (target == this.children[1]) {
				var elem = this.children[2].rows[0].cells[0];
			}
			elem.style.background = target.value;
		}
	}
}
</script>