Hair color is different from strain to strain...for example White Widow is harvested with hairs just starting to turn auburn (approx. 60 days), and others are ready when the hairs are dirt brown...hairs aren't a good indicator
unless you have intimate information with growing that strain, and have learned whats best. The trichs are the best way to tell. There are 3 levels of maturity with the trichs; immature (clear), peak (cloudy, or milky), and mature (amber). So go grab a 50x-100x magnifying glass, and check those trichs on the buds, harvest your ladies when you see a 50/50 mix of clear, and cloudy. This will net you the most THC