You might want to flip back to the last pic as you read this but i needed a really close shot so you can see these lines.

Start a new layer on top of all the layers you have. "dark highlights" Set your brush to size 1 with an opacity of 70% and flow 67% using the darkest color, make small lines here and there in groups of 3-4 around the hair with the pen tool system.

Start another new layer on top of all the layers you have. "light highlights1" Set your brush to size 2 with an opacity of 70% and flow 67% using a lighter color than the base hair, make small lines here and there in groups of 2-3 around the hair with the pen tool system.

Make a final layer on top of all the layers you have. "light highlights2"Set your brush to size 1 with an opacity of 70% and flow 67% using an even lighter than the last layer, make small lines here and there in groups of 3-4 around the hair with the pen tool system.

Yay your done.