The really easy way is to just paint the hair in and use blend modes. You'll have a lot of control doing it that way.
For example, open a pic of someone's hair in
ps, then sample its colour - if you wanted something which would be somewhat similar to the base colour of the person's hair, or, pick a totally different colour if you wanted something really bizarre-looking.
Add a new empty layer on top of all and change its blend mode to something like soft light, colour burn, or whatever it is you wanted to try (because you can always change its blend mode as you go along), then begin painting on the new layer. Change the blend mode to see what different blend modes will do with the paint you applied, and if you don't like the results, change the blend mode, and play with the opacity of the layer, or, delete the layer and start over again.
It's quick and easy.
Mig