Hey! If you're interested in learning how to change hair-color using Adobe Photoshop, here is a simple way to do it!
STEP 1: GET A PHOTO (with a white background)
I took the photo above in my garage. The light is reflecting off of the white garage door, brighter than the other lights in the room, creating a washed out effect behind the subject. You can use my photo for practice!
STEP 2: COLOR THE HAIR (with a brush tool)
Create a new layer. (I will refer to this as your "Color" layer.) Use the brush tool (shift + b) to paint the colors you want over the hair. Use a soft brush If you want to blend different colors together.
Apply your color(s), completely covering only the parts of the hair you want to add color. It is a good idea to try this a couple of times on different layers. After a couple tries, choose your favorite layer and hide the others. Keep in mind, the final colors may turn out different than the colors you intended.
Hint: Any color that is covering your white background will not be visible in your final image. Don't worry about staying in the lines! Erase any color that is touching the skin.
STEP 3: Change Opacity and Layer Style
Lower the Opacity of the Color layer to 60%
Then change the layer style of the same Color layer from Normal.. To Overlay!
WOW! Look at that! It's like MAGIC. I know.
Some Final Tips: Play around with your image, using layers and layer masks to create different effects. Use brighter colors because they usually end up looking less saturated. If you want less saturated colors, lower the opacity. If you want even brighter colors, duplicate the Color layer and keep the opacity at 60% or below for both layers.