Puoi fare cosi

$arr_background contiene la lista dei colori dello sfondo
$arr_color contiene la lista dei colori del testo che corrispondono ai sfondi

codice:
<?php 

srand ((double) microtime() * 10000000);

$arr_background = array ("blue", "yellow", "orange", "green", "red");
$arr_color      = array ("white", "red", "black", "blue", "white");
$rand_keys      = array_rand ($arr_background);
$background     = $arr_background[$rand_keys];
$color          = $arr_color[$rand_keys];
?>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>

<title>Untitled</title>
<style type="text/css">
<!--
input{color:#000000;}
a:link{color:#4682E3;}
a:visited{color:#4682E3;}
div.a{background-color:#4682E3;}
body{background-color:<?php print $background; ?>; color:<?php print $color; ?>; font-family:Comic Sans MS;}
-->
</style>

<body><center>
<title>MyBox MeGacHaT</title>
<hr><div class=a><font color=white>MeNu principale</font></div><hr>

pippo
</head>
<body>

</body>
</html>