To draw hair, I basically use three tools: AirBrush, Blur and Eraser. :) I'll show you how did I make the hair for this drawing, step by step:
1. I start with a base, so hard outlines of the head won't be visible through the hair.
2. Using a color close to the target color of the hair, I draw the first layer of hair with thick but soft strokes. Those lines will be my reference, when I'll be working on the details later.
3. Using slightly thinner lines, I draw the second layer of hair. This time I used a darker color. I try to keep the shape I made with the first layer.
4. I use the approximate hair color again and I draw another layer of hair (with even thinner brush), basing on the first layer.
Zoom on details:
5. I repeat the last step with an even thinner brush.
Zoom:
6. I put shadows on top of the head and on bigger "arcs" (such as the bangs) with a really big brush. Dark color of course.
7. Same color but smaller brush - shadows and curls.
8. I put the highlights the same way I do with shadows - large soft brush. Then I blend the highlights with the blur tool.
9. Finally I erase the highlights and shadows that are outside the hair, and I "cut" the bangs. :)
The "back hair" are made the same way, so I won't explain that, because I'd repeat myself. :)
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PL: http://nimfpa.blogspot.com/2015/03/malarstwo-cyfrowe-digital-painting-wosy.html