Painting realistic hair is a very difficult thing to do. Mastering this skill is a matter of possessing both theoretical and technical skills. Moreover, different kinds of hair require different levels of skill – painting straight hair is much easier than depicting curls or waves. When you’re painting curls you have to think in 3D, and work out not just how the hair falls, but also how the curls will interact and how they’ll be affected by the hairstyle.
Colour is another area that can cause problems. While the vast majority of painting subjects require a simple palette consisting of one midtone and some accompanying shades, hair’s natural colour diversity requires simultaneous work with a few midtones.