The following script takes the path of an image and a color value to set to Alpha. Then the image takes as background the $bgcolor. A final parameter $mirror is used to mirror the image horizontally.

gdImage_manipulator.php

<?php

/*
 * The callback function to process the image.
 *
 * @param string $filepath
 *   The file to the path.
 * @param string $color
 *   The HEX value of the color.
 * @param string $bgcolor
 *   The HEX value of the bgcolor.
 * @param bool $mirror
 *   Set to TRUE to to mirror the image. Defaults to FALSE.
 *
 * @return string
 *   The PNG's image data.
 */
function gdimagemanipulator_imageprocess($filepath, $color = '000000', $bgcolor = 'FFFFFF', $mirror = FALSE) {
  $image = imagecreatefrompng($filepath);
  $width = imagesx($image);
  $height = imagesy($image);

  // Foreground color.
  $color = array(
    'R' => hexdec(substr($color, 0, 2)), // Get the RED color for the front.
    'G' => hexdec(substr($color, 2, 2)), // Get the GREEN color for the front.
    'B' => hexdec(substr($color, 4, 2)), // Get the BLUE color for the front,
  );
  $fg = imagecolorallocatealpha($image, $color['R'], $color['G'], $color['B'], 0);
  imagefilter($image, IMG_FILTER_GRAYSCALE);
  imagefilter($image, IMG_FILTER_COLORIZE, $color['R'], $color['G'], $color['B'], 0);

  /*
   * The following commented part of code is use to provide the image with
   * a white background color. But since we have multiple images, this is no longer
   * needed. It is only needed right not to give a transparent background;
   */
  $bgcolor = array(
    // Get the RED color for the bg.
    'R' => hexdec(substr($bgcolor, 0, 2)),
    // Get the GREEN color for the bg.
    'G' => hexdec(substr($bgcolor, 2, 2)),
    // Get the BLUE color for the bg.
    'B' => hexdec(substr($bgcolor, 4, 2)),
  );
  $image2 = imagecreatetruecolor($width, $height);
  // To allow proper background color you might have to comment out the
  // following two lines [...]
  imagealphablending($image2,false);
  imagesavealpha($image2,true);
  // [...] And uncomment the next two lines to allow proper bgcolor.
  //$bg = imagecolorallocatealpha($image2, $bgcolor['R'], $bgcolor['G'], $bgcolor['B'], 0); //set bg
  //imagefill($image2,0,0,$bg);

  // <ix the two pictures.
  imagecopyresampled($image2, $image, 0, 0, 0, 0, $width, $height, $width, $height);

  // Check if we should mirror image.
  if($mirror) {
    _gdimagemanipulator_imageprocess_flip($image2);
  }

  /*
   * Change the following code to match your return needs.
   */
  // Display processed image.
  drupal_set_header('Content-Type: image/png');
  $output = imagepng($image2);
  imagedestroy($image);
  imagedestroy($image2);
  return $output;
}

/*
 * Function to flip an image with GD.

 * ImageMagick has a built-in function but with GD, We have to create one
 * and with all these dependenciesthat ImageMagick has, this is a no-go.
 */
function _gdimagemanipulator_imageprocess_flip(&$image, $x = 0, $y = 0, $width = null, $height = null){
  if ($width  < 1) {
    $width  = imagesx($image);
  }
  if ($height < 1) {
    $height = imagesy($image);
  }

  // Truecolor provides better results, if possible.
  if (function_exists('imageistruecolor') && imageistruecolor($image)) {
    $tmp = imagecreatetruecolor(1, $height);
  }
  else {
    $tmp = imagecreate(1, $height);
  }
  $x2 = $x + $width - 1;
  for ($i = (int) floor(($width - 1) / 2); $i >= 0; $i--){
    // Backup right stripe.
    imagecopy($tmp,   $image, 0, 0,  $x2 - $i, $y, 1, $height);
    // Copy left stripe to the right.
    imagecopy($image, $image, $x2 - $i, $y, $x + $i, $y, 1, $height);
    // Copy backuped right stripe to the left.
    imagecopy($image, $tmp, $x + $i, $y, 0, 0, 1, $height);
  }
  imagedestroy($tmp);
  return true;
}