There is a really good tutorial here: http://handcannononline.com/blog/2011/04/20/tutorial-basics-%E2%80%93-glorious-locks-painting-hair/
My way of painting dark brown hair is as follows.
1)Undercoat Scorched Brown.
2)Mix a touch of water, and a touch of Scorched Brown into Badab Black wash. This bit is a bit of trial and error, I look for a watery wash that is dark brown almost black. Use this and try and paint it into any recesses in the hair.
3)Go over any raised parts again with Scorched Brown.
4)Mix a bit of Bestial Brown with Scorched brown so you get a brown between the two and use this to paint just the top of any raised hair where you think the light will catch it.
5) Mix Bestial Brown and Snakebite Leather with a little bit of Scorched Brown to get a lighter and use that even less on raised areas where the light would reflect off that most.
6) If you are doing a female character and you want shiny glossy hair (like on those Pantene shampoo adverts!) a final highlight of snakebite leather only can be use to make more shine.
The most important thing with this is that you need to have quite thinned with water paints. This lets your previous colours shown through so the transition between colours isn't so abrupt. What I described won't win you any Golden Deamons but I think it looks really good and effective. As an example I did what I descried above to do the hair on Perdita - http://www.coolminiornot.com/282117