Go to the Flats channel and use the Marquee Wand Tool to select an entire area of hair.

Lower the Tolerance of the selection at about 10, to minimize overlapping the colors.

Go back and select the Color layer, and pick the color the same as the one you used as a base.

Pick the Gradient Tool , use Foreground to Transparent: Straight Gradient.

Set the mode to Multiply, Opacity about 50%.



Start dragging the tool to the direction you see on the left, making sure you leave enough of the base color in the middle to indicate highlights.