Choose your paintbrush and set the 'Mode' to 'Hue'. Use the eyedropper tool to select a color from the hair of our model that is just a bit on the dark side of the various colors found in the hair.
Run over the edges of the hair and the green will become the shade of the hair. Be careful - don't run over any areas other than the hair with this color loaded. If you need to touch up other areas, use the eyedropper to load the appropriate color first (for example, is we wanted to clean up the shirt, we would first pick the grey color of the shirt and then do the Hue brush).

(Run over just he edges, adjust the opacity down to around 60%)

Here's Hedda, cut out and a photo background placed on a new layer behind her.
The more time you spend on this, the better your result will be. Practice make perfect.
To soften the edges, use the blur tool set with a very subtle opacity and run around the edges a few times. Once again being careful not to blow out the edges to bad.

Here's a closeup of the hair in the left corner after knocking out and blurring slightly. Not bad! I hope you give this technique a try. Don't get frustrated, if you mess it up, start over again making sure of every step.