Now make a new raster layer (layers > new raster layer) and fill it with any color you want. I'm using the color #AEFE22. After you fill it with that color set the raster layer to Color (L)
Now make another new raster layer and fill it with any dark grey color. Then set that layer to Saturation (L)
After that get out your eraser tool and erase any parts of the picture that you want the green to show through. For instance, erase her lips and eyes and you can see the green appear. Mine turned out like this:
You can try this effect with different colors and gradients :) Be creative!