It depends a bit on the hair style but generally:
Select the hair.
Go to the surfaces tab.
Generally the base colour of the hair is an image.
If you have blonde hair selected you can try to change the colour to the right of the image
You can also select an image of your own
Go to the surfaces tab again
Click on the image
Browse
Select an image of your own (generally I adjust the colour of the image provided in an image editor and save it under a new name)
Third way is to go to the surfaces tab and select the hairstyle
Then go to shaders
Iray
Hair
Select a hairshader you bought (generally any colour you want)
I made this character using the second method (there is a little purple in her hair)
The character with blue hair has a different hair texture (third method):
and lastly I made two of these characters using method two and three: