Changing the hair color of a person in photoshop is not fundamentally different from changing the color of anything. If you want to change the color of an area in photoshop, you have to define it first. You define an area in photoshop by selecting it. Depending on how uniform or complex the area is you have different selection tools (magic wand, lasso, marquee...). But to me, the tool that provides the most flexibility is the pen tool, because it allows you to define an area first and then make the necessary adjustments using the + or - or direct selection. After you have defined the selection area with the pen, you have to turn the work path you have just created into a selection. To avoid sharp edges in the color change, you need to add a feather value or 1 or 2. After that, you can go to Hue/Saturation or Variation to play with it until you get the desired results. Of course you can create an adjustment layer and use different blending modes until you find the one that suits you best.