Blending colors GD - PHP
$image = 'o1.png';
$overlay = 'o2.png';

$background = imagecreatefrompng($image);

imagealphablending($background, true);

// Create overlay image
$overlay = imagecreatefrompng($overlay);

// get size
$size = getimagesize("o2.png");
$L=$size[0];
$H=$size[1];

for($j=0;$j<$H;$j++){
for($i=0;$i<$L;$i++){

$rgb = imagecolorat($overlay, $i, $j);

$red = (isset($_GET['r']) ? $_GET['r'] : 0);
$green = (isset($_GET['g']) ? $_GET['g'] : 0);
$blue = (isset($_GET['b']) ? $_GET['b'] : 0);

$r = ($rgb >> 16) & 0xFF;
$g = ($rgb >> 8) & 0xFF;
$b = $rgb & 0xFF;

if(($r==255)&&($g==255)&&($b==0)) {

$color = imagecolorallocate($background, $red, $green, $blue);
imagesetpixel($background, $i, $j, $color);

}

}
}


header("Content-type: image/png");
header("Content-Disposition: filename=" . $image);

imagepng($background);

// Destroy the images
imagedestroy($background);
imagedestroy($overlay);