Colour Masks (Probabilistic Maps) - 2017
Blending colours in a realistic way is quite complex and needs a very detailed texture map in order to mimic nature paterns.
One option is, instead of paint textures on an external software, to paint attributes directly in Houdini, one for each colour. Those colours are blended together per hair using probabilities.
For example, if we have a black mask and another white mask, if black has a 0.7 value and white 0.3, this means that in that area 70% of the hairs will be black and 30% of the hairs will be white. A texture would make the hairs in this area grey.
Painting those colour masks is very fast compared to the traditional texture painting, specially when we have to create different colour patterns.