Kind of a tutorial; I've never been good at explanations but I think this is pretty straight forward.
I always use a five color palette for hair, but I don't always use the addition layers it shows at the end (they're more there for fun, especially the purple one)
and it's hard to tell on there but I turn the layer opacity lock on and off and use the eraser to keep from drawing over he face (the hair layer is over the lineart and everything else)
Hope it's a bit helpful and just ask if you don't get something or whatever, I don't bite.
Program is SAI but I know this works in at least OpenCanvas, as well and it should work most anywhere. Yes I have a tablet, but I can do this with my touchpad (it just takes longer) so it should be applicable with a bit of work to mouse users (I don't have an actual mouse or I would try).