How to set multiple transparent colors using imagecolorallocate()

[Remix919]

Hi there, I'm trying to make a watermark using a JPG background file and a JPG logo file. Now, I want certain colors of the logo to be transparent, namely shades of green. Anyone know how to do this? Here is my code I am using:

$green = imagecolorallocate($jpgimage, 0, 255, 1);

imagecolortransparent($jpgimage, $green);

 

imagecopymerge($img, $jpgimage, 100, 200, 0, 0, 640, 480, 100);

 

And that code works fine, but it only makes the $green RGB transparent, how can I make more than just that one RGB transparent? I would ideally want to make all shades of green to appear transparent.

[Remix919]

bump~