There is so much out there now, and most if it is pretty good info.
Testing can not be stressed enough and allowing them to "age and oxidize" if possible.
Full matrix coloration is the only way to go, as any "surface treatment" is subject to degradation.
Hair additives are more applicable to internal plasters than to external renders. Most Undulate like deer, elk, moose, have hollow hair that is insulative...yes...but very brittle. Hog hair is much more robust and seems to resist the alkalinity of lime much better...Again, experiment and share your results. Plant fiber is the over all best fiber as is additives like "rice soup" which is an ancient additive to lime and earth plasters. This is seldom (only a few of us have written of this method in English) is well known and documented but the formulations are handed down to orally, and any in depth studies are mainly in Chinese.
I could continue for pages on the many amalgamations of lime - earth plasters, and there pigmenting, but a good Googling will get most folks what they need.
Good luck, and please post pictures!