Our smudge layer works as a nice base for more detail work. Make a new layer on top of our first one. With the settings you see in the image I go about repainting strands. Once more, tons of strokes to work out those strands and hairs. You can actually reuse the color you used for the last step before the smudging or you can go and use a slightly brighter version of the previous one. The strands you paint now will automatically appear to be on top of the base we made earlier due to the fact that they are sharper then the smudged layer beneath it, for now. What we are doing here is creating some depth of field which is probably good to see at the closeup in the image and the ends of her hair close to her chin. Normally you would smoothen those endings out to not make them look as harsh and stubbly but I wanted to go for a roughly cut look, so I left it quite harsh.

There are several ways of going about to render the tips of hairstrands or free flowing endings. You can either smudge them out carefully, use the airbrush tool or use a little trick that works for me for hair which is further back on the head: Simply colorpick the background color and use a huge brushtip with very low flow. Then go about painting very carefully and slightly over the parts you want to tone down a bit. That also works for most elements that lay further back in your painting. It's however a very rough and messy way of creating some depths.

Now what? Well, we are in the rinse and repeat stage :D

Flatten the hair layer and smudge it out again (bored yet?). From here it's basically the same again: Create a new layer, pick a brighter color, work out more strands (strokes, strokes and more strokes) and lay them on top of each other. Color pick from the whole range of values you have in your hair now. If you have to, even go and deepen the shadow parts with several single strokes but be careful with that. You'll notice, that by now you should have a quite huge range of different color values (this is probably a strange example since she doesn't really have what I would call "normal" colored hair.)

Depending on how much patience you have, you can go on as long as you like. The more you do the more realistic it will look.
Before the final step of this painting I picked another highlight color (in this case ALMOST white - it's not a pure white though but a shade of the main lightsource color) and put focus on some special strands.
Don’t forget to vary both your brush tip and the flow and pressure you use while painting A LOT. It helps creating some dynamic in your hair.
This rule, by the way, applies to painting in general.

To finish up – carefully smudge out endings and parts that lay further back and go over parts that are in focus with a sharp edged brush one last time.

Voilà, after some more time and a bit too much oversharpening (I would actually go back and change quite a few things on this but it works for now ;))