For brown hair I start with Scorched Brown. If you want it darker I recommend mixing any type of red or darker yellow with the Scorched Brown until you get to a good tone. I personally haven\'t experimented much with darker hair.
For lighter hair you could mix a lighter/medium yellow like Golden Yellow into the Scorched Brown and keep highlighting until you find a good point to stop.
If you\'re finding yourself low on time then you can just use the pre-mixed paints from the GW line. Dark Flesh over Scorched Brown makes for a good darker hair, and Bestial Brown, then Snakebite leather, and possibly Bubonic Brown are good for lighter hair.
Hope this helped =D