I suspected that the new ZBrush might be better at all kinds of 2D painting, including hair, and it’s true! Perhaps it is not quite as powerful as Photoshop with its 2D layers, layer masking, and drop shadows, which allow for some very nice hair painting, but I hit upon a combination in ZBrush that is very fast, and without all the ‘options’ of Photoshop, I’m actually much faster at painting hair in ZBrush.
Use the DirectionalBrush tool, Alpha ‘Brush 07’, and the Freehand Stroke. No need to make any changes of tools or alphas during the painting process.
I selected a base color (red-ish), a ‘shadow’ color, and some ‘hightlight’ colors. I painted swatches of these colors so that I could quickly change back and forth by moving the cursor over the swatch and pressing ‘c’. Of course, you can also pick colors out of the hair that you have already painted using the same method.
The only tool modification you need to make during painting is to move the Focal Shift and Draw Size back and forth a bit. I started with a negative focal shift, which is analogous to using a thick brush to block in basic values/forms. For the blocking stage, I used a Draw Size of 6 to 12 and a Focal Shift of -30.
Then for the next stage, use a Draw Size of about 25 to 35 and a high, positive Focal Shift. For fine hairs, I went up to 95, but for this stage, use something lower to start giving ‘hair texture’ to the painting. Start with the shadow color and then switch back and forth between the base color and the shadow color.
Then set Focal Shift higher and start alternating between the shadow color, the base color, and the highlights.
I went a little wild with fly-away hairs here. I’m sure it would look better with half as many, but I was testing, not trying to make the perfect coif.