Once all the pistils have gone brown, THC is rapidly breaking down into CBD and CBN, giving you buds that have a sleepy body stone with a diminished THC head high effect.
In future, try harvesting when the pistils are 95% brown, this will give you buds with peak THC levels and the most psychoactive effect.
If you prefer a heavy body stone, harvest when all the pistils have gone brown.
Deciding on harvest maturity solely judging by trichome colour is, in my opinion, not a reliable indication of trichome composition. Some strains will never have amber trichomes and others will have 75% amber after only 4 weeks of flowering. Waiting extremely long past 100% brown pistils for amber trichomes to appear can result in ending up with second rate buds, due to nearly all of the THC having broken down into CBD and CBN, two non-psychoactive cannabinoids.
Once all the pistils have turned brown, the plant is nearing death and a nearly dead plant can not support prime quality trichomes.
Peak cannabinoid quality, with the most THC and the most fully developed terpenes occurs when there are 90-95% brown pistils.
Pistil colour is a more precise indication of plant health, and it is plant health that determines cannabinoid quality, not trichome colour on its own.
I would say harvest ASAP, before even more of the remaining THC breaks down into non-psychoactive compounds.
Hope this helps,............
Organoman.