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.