An oil based hair brush to help with those struggles of wanting a realisim look. Can be used even for manga based purposes.
An example of this is
To use this brush as you like i recommend lowering the opacity to any layer you want but also having a dark color used first then build your way into the direction and flow of the hair nicely.
Notice my settings are at opacity 81% with a bit of color mixing and harndess is rough to create the hair strength color.
Finally, after creating the settings to how you like try to stroke the brush slightly already on colored hair (the base color), then continue the stroke while the layer you use if on lock for now. It takes me about 3-4 times of going over to create this look but i like to use the smaller sizes, if you used the bigger size it's less time consuming.