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This tutorial is intended to teach new GIMP users how to remove a green color cast from a picture of a leaf :)
First, as always, open the latest version of GIMP.
We'll be using this image, again from Lenspiration user snmiller
Notice the greenish cast on the background, that's what we're aiming to remove.
Open your image in GIMP and go to "Colors > Curves"
Now select the "Green" color curve option.
Now grab the line right in the middle and pull it down a bit, you can adjust it till it looks about right.
Voila! Green cast is all gone! Note that it does affect the other colors slightly, but I'm assuming that if the rock was "off" then the rest of the picture was too, so the whole thing should be more realistic. You may want to back down slightly on the saturation, since modifying the green channel basically eliminated the need for extra saturation.
Here's a before and after comparison...
