I don't think this effect already existed (maybe I din't search enough), so I wrote this little effect today.
Basically, this takes an opaque image along with a background/transparent color, and makes the image transparent in respect of the chosen color.
The closer a pixel's color is to the chosen color, the more transparent it will become. This can be used, for example, to change the background color of an image.
I took this image as an example:
Then I applied my filter choosing some random blue color I could find in the sky of the picture:
Here the image is really transparent (you might want to open it in Paint.NET to see better) and can blend really well on any background.
Now I applied a dark background (just by adding a layer under the image, and filling it with the color) and the image looks like if it was the night
This is still really simple, and you don't have any other parameter than the color yet.
I hosted the plugin on my ftp here: [Download]
If you have any questions or comment...