Go back to the Flats Channel, and again, select a whole section of hair.

This time we're going to add the Highlights.

Using the Brush Tool , this time changing its' mode to Screen, same Opacity level.

Pick a fairly light color from the base, like this.

Start brushing the area with a rounded stroke, following the angle and shape of the head.

For more detailed highlights, you can choose to select just a clump at a time, and add highlights that way. For the purpose of this tutorial, I will just use this general technique.