Adobe Tutorial

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