function turn_on_the_light()
{
	alert('');
	var mmenu=document.getelementbyid("main_menu");
	mmenu.style.bgcolor="red";
	
}
......
<td width="20%" height="80%" id="main_menu">
......
<tr><td align="center" onclick="turn_on_the_light()">Гостевая</td></tr>