Código PHP:
<title>DATOS DEL PREDIO</title>
</head>
<body>
<FORM ACTION="prueba.php" METHOD="POST">
<script>
$(document).ready(function(){
$("#buttonEnviar").click(function(){
$.post("prueba.php",
{valueSelect: $("select").val()},
function(json){
if(json.msg=="ok"){
alert("variable guardada");
}
},"json");
})
})
</script>
<SELECT NAME="Colores" MULTIPLE>
<OPTION VALUE="r">Rojo</OPTION>
<OPTION VALUE="g">Verde</OPTION>
<OPTION VALUE="b">Azul</OPTION>
</SELECT>
<img src="img/enviar.bmp" style="cursor:pointer" onclick="submit();"id="buttonEnviar" >
<?php
$_SESSION["Colores"]=$_POST["valorSelect"];
$varcolor=$_SESSION["Colores"];
echo "$varcolor";
?>
</body>
</html>