Yo lo haría de esta manera:
Código PHP:
<html>
<head>
<script language="JavaScript">
var aBackground = new Array("uno.gif",
"dos.gif",
"tres.gif",
"cuatro.gif",
"cinco.gif",
"seis.gif",
"siete.gif",
"ocho.gif",
"nueve.gif",
"diez.gif",
"once.gif",
"doce.gif",
"trece.gif",
"catorce.gif",
"quince.gif",
"dieciseis.gif",
"diecisiete.gif",
"dieciocho.gif",
"diecinueve.gif",
"veinte.gif",
"veintiuno.gif",
"veintidos.gif",
"veintitres.gif" );
var aBackColor = new Array("blue",
"aquamarine",
"chocolate",
"darkred",
"gold",
"red",
"yellow",
"hotpink",
"lime",
"darkkhaki",
"cadetblue",
"darkgoldenrod",
"darkslateblue",
"blue",
"deeppink",
"darksalmon",
"salmon",
"tan",
"wheat",
"tomato",
"springgreen",
"turquoise",
"white");
function cambia(){
document.bgColor = aBackColor[document.frm.clr.selectedIndex];
document.body.background = aBackground[document.frm.clr.selectedIndex];
}
</script>
</head>
<body>
<form name="frm">
<p>
<select Size="5" name="clr" onChange="cambia()">
<option VALUE="blue">Azul </option>
<option VALUE="aquamarine">Azul Agua </option>
<option VALUE="chocolate">chocolate </option>
<option VALUE="darkred">Marron </option>
<option VALUE="gold">Oro </option>
<option VALUE="red">Rojo </option>
<option VALUE="yellow">Amarillo </option>
<option VALUE="hotpink">Rosa Fuerte </option>
<option VALUE="lime">Lima </option>
<option VALUE="darkkhaki">Verdoso </option>
<option VALUE="cadetblue ">Lila </option>
<option VALUE="darkgoldenrod">Oro Viejo </option>
<option VALUE="darkslateblue">Azul Oscuro </option>
<option VALUE="blue">Azul </option>
<option VALUE="deeppink">Rosa Chillon </option>
<option VALUE="darksalmon">Salmon FLojo </option>
<option VALUE="salmon">Salmon </option>
<option VALUE="tan">Café Claro </option>
<option VALUE="wheat">Trigo </option>
<option VALUE="tomato">Tomate </option>
<option VALUE="springgreen">Verde Fluorescente </option>
<option VALUE="turquoise">Turquesa </option>
<option VALUE="white" SELECTED>Blanco </option>
</select>
</p>
</form>
</body>
</html>
Saludos.