invert.php

#!/usr/bin/env php
<?php
$content = file_get_contents($argv[1]);

function invert_color($matches) {
  $r = 255 - intval(substr($matches[1], 0, 2), 16);
  $g = 255 - intval(substr($matches[1], 2, 2), 16);
  $b = 255 - intval(substr($matches[1], 4, 2), 16);
  $a = isset($matches[2]) ? $matches[2] : '';
  
  return sprintf("#%02x%02x%02x%s", $r, $g, $b, $a);
}

$inverted = preg_replace_callback('@\#([a-f0-9]{6})([a-f0-9]{2})?@i', 'invert_color', $content);
file_put_contents($argv[1] . '.inv', $inverted);