In this tutorial Ill Show how you can easily change someone hair color.
Step 1
Open your photo.
I chose this one:
Use quick mask to select the hair: press Q to start using quick mask.
Take a big hard brush (hardness and Opacity set to 100% ) with the color black, And paint with it over the hair. Dont worry about getting out of lines a little, well take care for that later. When you paint with black using quick mask anything youll paint will be red.
Step 2
When you finished masking all the hair Press Q again, and youll get a selection of anything except the masked area.
Press Ctrl + Shift + I To select Inverse, and youll have a selection of the hair.
Step 3
Make a New layer By pressing Ctrl + Shift + N or going to Layer > New Layer.
Set the layers Blend mode to Soft light
Choose the color you want to use and go to Edit > Fill To fill your selection with the color.
The color you choose at this point doesnt matter much because you can easily change it later, But try to choose a color similar to the one you want in the end result.
I want to make her blond so I chose this color: #aca177
After you filled in the layer press Ctrl + D To Deselect.
Step 4
Zooming in the photo you can see that there are many places that shouldnt be colored.
Take the Eraser Tool
Choose a brush with its Hardness Set to O, and Opacity about 50%
And delete or soften all the areas that shouldnt be colored
Step 5
After you finished deleting all the unwanted areas,
Right click on the color layer and choose Duplicate Layer
Zoom in, and youll see some new parts that should be cleaned,
Remove them with the Eraser Tool
This is my result:
Step 6
If you dont like the color of your end result, you can easily change it.
In the layer menu, Click on the Lock Transparent Pixels Icon.
Do it on both hair color layers.
Now all the transparent pixels cant be colored.
Choose a color, and go to Edit > Fill.
Do it on both layers.
You can also choose different color for each layer for a better result, I think when coloring a dark hair its better you use a darker color on the first layer and a brighter color on the second layer.

Here some examples I made, Along with the colors I used.

Second layer color: #ebebcd

First layer color: #092336

Second layer color: #6692b0

First layer color: #5e2e11

Second layer color: #eaaa87

Before we can do anything with a Black & White adjustment layer, we first need to add one. Theres a couple of ways we can do that. One is by clicking on the New Adjustment Layer icon at the bottom of the Layers panel:

To bring color back to the image, change the blend mode of the adjustment layer from Normal to Soft Light. The Blend Mode option is found in the top left corner of the Layers panel:

The color returns to the photo. You may already notice a change in the overall contrast of the image, but well make our own adjustments in the next step:

If youre using Photoshop CS3, the controls and options for the Black & White adjustment layer will appear in a dialog box. In Photoshop CS4 and higher, they appear in the Adjustments panel. In either case, youll see six sliders, each controlling a different color in the image (from top to bottom - Reds, Yellows, Greens, Cyans, Blues, and Magentas):

If youre using Photoshop CS4 or higher, its even easier to make changes to the image. In fact, theres no need to use the sliders at all! Just click on the slider icon (the hand with the left and right-pointing arrows) in the top left of the Adjustments panel:

With this slider option enabled, you can click directly on whatever area of your image you want to adjust, then just keep your mouse button held down and drag left or right to darken or lighten that area (along with any other areas of the image that share the same color).

For example, to make the horses in my photo appear brighter, with the slider option enabled, Ill move my mouse cursor over one of the horses (Ill use the one on the right). Since both horses are the same color, brightening one of them will brighten the other at the same time. As soon as you move your mouse cursor over the image, it will turn into an eyedropper icon, ready to sample whatever color you click on:

Ill click on the horse and, with my mouse button held down, Ill drag towards the right to lighten the horse, as well as any other areas of the image that share the same color (the other horse on the left, plus the red barn in the background). If you watch the sliders in the Adjustments panel, youll see the corresponding color slider moving as you drag your mouse (in my case here, the Reds slider would be moving to the right):

Since were using an adjustment layer, all of the changes were making here are non-destructive, which means we can safely experiment with the color sliders, or by clicking and dragging directly on the image itself, until were happy with the results. Earlier I lightened the grass and trees, but if I want to see how the image would look with the grass and trees darker instead of lighter, I can just click anywhere on the grass or trees in the image and drag my mouse towards the left:

If, at any time, you want to compare the original and edited versions of the image, simply click on the adjustment layers visibility icon (the eyeball icon). This will temporarily turn the Black & White layer off:

With the adjustment layer turned off, the original image re-appears in the document window:

Click on the visibility icon again (the empty box) to turn the Black & White adjustment layer back on:

The edited version re-appears:

For Photoshop CS3 users, click OK in the top right corner of the Black & White dialog box when youre done to close out of it and accept your changes. Photoshop CS4 (and higher) users can leave the Adjustments panel open.

As a final step, if you think the image contrast is now a bit too strong, you can reduce it and fine-tune the results simply by lowering the opacity of the Black & White adjustment layer. Youll find the Opacity option directly across from the Blend Mode option at the top of the Layers panel. The further you lower the opacity value, the more the original image below the adjustment layer will show through. Im going to lower my opacity value down to 75% or so:

Heres my final result:

 And there we have it! Thats how to quickly and easily make specific brightness and contrast changes to a full color image using a Black & White adjustment layer!