The reddening of the pistil hairs is an early warning sign that she is getting closer to harvest but it isn't a real good precise measure because as those pistils get old and die off, more are grown out to replace them that are white again. Once that process of replacing the pistils with new slows down (the buds should be fairly fat from the new calyxes growing), That is a better sign that you are very close and it is time to begin looking in earnest at the trichomes.
However, this trait doesn't hold entirely true for all strains. There is some variation there so you have to watch carefully and not just trust one thing.