This script takes two hex values and produces the average color.

colormixer.php

<?php
	$hex1 = 'FF0000';
	$hex2 = '0000FF';
	$hex3 = '';
	
	$r1 = base_convert($hex1{0}.$hex1{1},16,10);
	$g1 = base_convert($hex1{2}.$hex1{3},16,10);
	$b1 = base_convert($hex1{4}.$hex1{5},16,10);
	
	$r2 = base_convert($hex2{0}.$hex2{1},16,10);
	$g2 = base_convert($hex2{2}.$hex2{3},16,10);
	$b2 = base_convert($hex2{4}.$hex2{5},16,10);
	
	$r3 = round(($r1 + $r2) / 2);
	$g3 = round(($g1 + $g2) / 2);
	$b3 = round(($b1 + $b2) / 2);
	
	$r3 = (string)base_convert($r3,10,16);
	$g3 = (string)base_convert($g3,10,16);
	$b3 = (string)base_convert($b3,10,16);
	
	if($r3 == 0) $r3 = '00';
	if($b3 == 0) $b3 = '00';
	if($g3 == 0) $g3 = '00';
	
	$hex3 = $r3.$g3.$b3;

?>

<div style="width:100px;height:100px;float:left;background-color:#<?php echo $hex1; ?>"><?php echo $hex1; ?></div>

<div style="float:left;font-size:72px;">&nbsp;&nbsp;&nbsp;+&nbsp;&nbsp;&nbsp;</div>

<div style="width:100px;height:100px;float:left;background-color:#<?php echo $hex2; ?>"><?php echo $hex2; ?></div>

<div style="float:left;font-size:72px;">&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;&nbsp;</div>

<div style="width:100px;height:100px;float:left;background-color:#<?php echo $hex3; ?>"><?php echo $hex3; ?></div>