The key here is observation — look at how each strand(or section) of hair moves, where they start, and where they end. Using the pencil draw each strand of hair from root to tip, following the same paths you observed. For these initial strokes use mid tones to shape the major hair sections and parts. Then gradually brighten the color as you draw in hair with highlights. Adding small strokes of white and light yellow can really bring life to the hair, just be careful not to over do it.
For those new to the app I suggest drawing in grays — limiting your palette to get a feel for the tools and how they behave. Working with a limitless selection of color could complicate and distract you from learning the tools. Thankfully FiftyThree’s excellent color mixer just works when you’re ready to exploit it.
On the subject of palettes… I stick with the default sets. Since I draw a ton of portraits, it’s much quicker for me to adjust a color on the fly. But I do see the benefit of setting up a predetermined palette beforehand to give your illustration a feeling of cohesion. Before the color mixer existed you had to get inventive if you wanted additional colors. A limitation that encouraged experimentation and sometimes unexpected results.
Working an area too much with the pencil can cause your hair to flatten out. To fix this, draw dark pencil strokes over the problematic areas. If you follow the hair paths, drawing root to tip again — these dark strokes can help add detail back to the hair. Make your strokes quick and try not to go over the same line twice or else it might darken too much.
If you’re a maniac with the pencil and prefer to draw in all the gradations then feel free to omit this step. I’m much too lazy for that and prefer to paint over the strokes to achieve the same effect. A warm light gray works great for brunettes and a light orange or pink for redheads. Experimentation is the key here. If you don’t like the result just two finger rewind that noise and try another color. Lighter layers of watercolor that are built up gradually allow for subtler tones than a stroke of black would.
PS: I wrote another guide called Mastering Paper by 53, that expands on all these techniques in greater detail.