I think I began to paint hair instinctively. When I was showing some of my portraits, someone asked me how to paint hair. I didn’t know how to answer the question, because I had never thought about it. The following information is an attempt to give some helpful ideas to paint beautiful and realistic hair.
When you look at a photograph of hair, you will notice that when light hits clean hair, it appears nearly white. So I try to think of the shapes of the areas where the light bounces off the hair, and leave those areas white. In effect, I am negative painting around the areas which have highlights. Very black hair often has a light blue highlight.
I think the most challenging color of hair to paint is blonde. To get a realistic color of hair, I 
may use light lavender, pale gold, pale tan, pale gray or even pale green or 
pink! Is the blond hair yellow or gold (warm), or a cool color? I always vary the color of the hair, as there are a great variety of colors of hair in one head of hair! Also, blond hair can have colors from reflected light. Some hair has little if any shine, and other hair is very shiny. Children’s hair tends to shine more than adult hair.
For brown hair, I often use burnt umber, and add some cobalt 
blue to darken it if needed. For a warmer color brown hair, use a warmer brown such as copper penny, or add a transparent red to the burnt umber. You will have to mix some samples to find the colors useful for your painting.
If you want to mask out a few strands, apply masking medium (rubber cement also works) with the edge of a fine 
palette knife, which will leave a more delicate line. If needed, you can use an Exacto knife to indicate a few strands of the hair against a darker background.
For me, painting animal fur is not much different than painting human hair. The variances 
are color, length and texture, however, highlights are needed to show the shape of the head or body beneath the hair, and highlights caused by the waves in or shine of the hair.
Marilyn Jacobson
Artist
11/25/2011