Okay. First, I started out with throwing down my base color. In this case, I lifted it right from TV show's palette, but you can use whatever color you want, and you don't actually have to lift it from another source. (Duh.
) Just try to pick a color that isn't blindingly bright. Neons, for example, wouldn't really work in a situation like this.As a side note, it helps to have strong lineart. I would suggest making sure that you have a few lines that show individual pieces of hair, a part, etc. If you don't have anything like that, then sometimes, half-way through coloring, you'll feel like something is off or that the hair feels "empty". Try to keep the lineart for the hair at the same level of detail as the other parts of the picture.
Also, after you get the base down and have cleaned up the excess brush strokes, lock the layer, and make a copy. (Turn the bottom layer off and keep it around as back up, just in case there's an unfortunate accident.)
2
Next, I went and laid down some preliminary shadows. I picked a slightly darker color than my base and used it where I knew the hair would appear darker. (The curl of his bangs, the hair too far out of reach from the light source, his part, etc.) Using that color, I also took a large brush at a low opacity and gently dabbed around the hair to build texture. You probably can't see it very well, but it should be subtle. If it looks like you've been dabbing with a big round brush when you're done... it's too dark.
(For some reason, you can see this more clearly in the next picture.)For my core highlights, I used the dodge tool* on a (very) low exposure and started by lighting dabbing it in places where the light source would hit, paying special attention to the right side of his head. After that, I went and used a smaller sized brush to start singling out individual strands of hair.
Lock the layer and make a copy.
*Dodge tool: The dodge tool makes things lighter. It can be toggled back and forth with the burn tool, which makes things darker. You can pick which types of dodge to use by using the drop-down menu next to brush size. I generally use highlight, as it leaves the greatest impact.
My rule for the highlight setting is that a large brush should be used for low exposures, and a small brush should be used for high exposures. This is very helpful in focusing in on a particular highlight. Change brushes often, and play around with it. (It's one of those things you just need to figure out on your own to make it work.) The same principles apply to the burn tool as well.
3
It probably looks like I did a lot in this step, but it's really pretty simple. I took a color even darker than my base shadow, and concentrate it towards areas that are supposed to be heavily shadowed. Then, using the smudge tool* with a small to medium brush size, I pushed the color outwards in the direction that the hair would fall, alternating back and forth between moving the dark colors out and the lighter colors in. This takes a bit of patience, but you can get really dramatic results when using a small brush. Feel free to add a second blast of color--choose an even darker shade and drop it at the core of the shadow. Continue to work outwards.
Lock the layer and make a copy.
*Smudge tool: Right next to the dodge tool. Can be toggled with "Sharpen" and "Soften". When you pick your brush size, always make sure the "hardness" is set to 0%. A hard smudge brush will leave streaks. It's ugly.
4
More highlighting. Use a large, low-exposure brush and pick out your highlights again. Then, use a small brush and delicately pick out individual strands of hair. (Again...) Alternate your exposures between high and low. Do the same for the burn tool, but focus it towards the shadows. Try and keep things subtle--if a part of the highlight/shadow sticks out in particular, you've probably gone to far. To negate a highlight, use the burn tool, set the brush to highlight, try to match the exposure, and simply go over the offending area. This works best in large areas. For small issues, simply smudge it out.
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And... that's it! I hope this was helpful. 
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