As far as I could tell an effect like this isn't represented here so I went ahead and just tore apart some existing effects by Shviller and Rojohound (thanks, guys) and reassembled them into this.
Basically what it does is: You pick a RGB colour and that colour becomes transparent.
<img src="http://i244.photobucket.com/albums/gg36/some9000/cd6dfc98.png" border="0">
<img src="http://i244.photobucket.com/albums/gg36/some9000/1ed90ca0.png" border="0">
<img src="http://i244.photobucket.com/albums/gg36/some9000/c50dc82e.png" border="0">
Really simple. Now WHY would you need this? In my case it helps with doing canvas capturing and then overlaying it on top of other things with transparency.
Grab it here.
Note: Some colours won't show as transparent in the editor, but all seems to work alright at runtime. Enjoy.