Some basic facts about painting hair which I have been gathering from my own observations or from studying theory:
Hair comes in layers
This means you have tons of single hair and strands laying on top of each other and like any other element like normal fabric for example, it casts shadows and interacts with and on itself. As a matter of fact I think clothing is in it's nature pretty close to how hair reacts - you can look at it as just some special kind of fabric some how..
Hair comes in Strands
Something very important to remember is that even though you might have single hair visible - they always clump together and cling onto each other..
Hair is messy
Even if you try to comb your hair fully straight - you will always have some single strands or hair which go the total opposite direction. Use this knowledge and be brave if you paint hair..
Hair has a depth of field like everything else
If you look at a photograph you'll notice that hair get's more blurry the further away from the focus it lies. That's actually general DoF theory but a lot of people keep forgetting to apply this to hair as well..
Hair colors are different then they seem (how to pick colors)
Â· Black hair has most of the time blueish/purplish/lilaish highlights or at least they appear like that.
Â· Red hair has extreme orange tints as highlights
Â· Never use pure yellow or gold for blonde hair. Use greyish yellows and cold brown tones instead - even greens or reds work.
Â· Hair is effected by your ambient color like everything else
In general it is hard to explain how you should go about picking colors for hair but there are some tricks I use:
Â· Always start with the darkest color (shadow) and go from there
Â· Use a brush and set it to low flow for starters. That will make the background color shine through slightly. Later on you can go and color pick from that color to establish more depth.
Â· Mix some of the skin color and background color into your hair (yet again low flow for the brush - so it mixes with the tones you already have set down if you don't want to pick the actual color)
Â· Brushwise - start BIG and end small.
Ok, now for an example. Explanation first, then you'll see the image which will hopefully help to make my muttering a bit more understandable.
Assuming you have a sketch or at least a general idea to work from, start to block in the main form of the hair with a dark color. Even blonde or bright brown hair as very dark shadows. If you use a darker and more saturated shade of the actual base color you should be off to a good start.
Don't worry too much about single strands or hair at this point but make sure to leave it pretty rough. Have some basic flow and directions in there, however.
I was working very big for this image which means I used a brush at low flow and 80 px tip (flow and size set to pen pressure). Like I said, if you work with low flow the background color will shine through a bit and leave you with some nice color values to work from.
I added some shape while switching between a lighter shade of the background color, some color I wanted to use for the skin later and a highlight color - but very roughly and lightly. The base color for this is a bit of an exception because that overall setting is very dark (night). Normally you would go and pick the shadow color of the main haircolor you want, to start with, like mentioned below
It is very important to go from darker to lighter colors as you work on the hair.
At this point I color picked a midtone from the color chaos I had from the first step and used it to slowly start defining the hair. I went in and started to detail the hair over her forehead a bit more with a smaller brush tip. I also went back in and used the background color on low flow to give her head including the hair a more roundish 3D feel.
Now you probably already noticed that naturally you got darker and lighter parts and hairstrands automatically start to come forth or fall back (even if you didn't plan on them). While color picking from the colors (dark and lighter part alike) you can go about to work this "accidents" out more to emphasize strands that lay on top of others.
At this point I started to pick the brightest color I had so far and started to paint in single strands (eventually you might want to adjust the color slightly after colorpicking.
Making it more or less saturated). This means I go about painting stroke by stroke until I have a strand of hair established (which basically takes a lot of patience).
Don't forget to totally change direction for some single hairs and don't go all over your hair but leave some parts darker or don't add a brighter color to certain areas at all (keep the lightsource in mind). Depending where your light comes from and depending on the volume of the hair, this is basically some knowledge you need to have (or rely on a picture if you have to).
Also make sure to vary the pressure you apply to your brush which is an easy way to work out hair volume. Start lightly and apply more pressure where the highest lit-by-light part would be and take back pressure as you go down or unto the shadow of a strand.
Once I have some basic hair/strands laid down I use the smudge tool and a special brush (see image) to smudge all over it again make sure you follow the hair flow while smudging though) which basically leaves me with a nice starting layer to work from. You will also notice that the brightest color will get toned down again while smudging - which is what we wanted to happen.