Not all plants change colour/fade, it all depends on how much nutrition they have received during flowering.
If you like couch lock, harvest when there are 100% brown pistils, this will put you in the (roughly) 30% amber trichome zone without having to look too closely.
If you prefer a more up high, harvest when there are about 95-98% brown pistils and this will put you in the (roughly) 10% amber trichome zone without having to look too closely.
Hope this helps, Organoman.