First step is to download the file "content\engine\engine_images.php"
Then you would edit the file "content\engine\engine_images.php" that you have just downloaded.
While editing that file look for the following line of code;
$white = imagecolorallocate($d, 255, 255, 255);
To make the actual change to the color, do so by changing the "RGB value" in the function.
For example:
For color FFCC00 yu would use;
$white = imagecolorallocate($d, 255, 204, 0);
Once you are done editing the file you should upload it back to the same place you downloaded it from overwriting the current file.
