Adobe Software Tutorial

                   

 Before  After

This tutorial showed how to change hair color in Photoshop. First it has you add a hue/saturation adjustment layer and select the colorize option in that dialog box. Then you can slide the saturation or hue to create the color of hair that you would like. After adjusting you will select the mask and hit ctrl + Backspace to fill the layer mask with black. Then you select the brush tool and make sure it is set on white as foreground color and black as the background color. Then you just use the brush to paint where you want the color to show through on the photo, and when there are loose strands lowering the opacity is good.

Source: 

http://www.photoshopessentials.com/photo-editing/hair-color/page-2.php