Some strains just don't develop amber trichomes, which is a genetic thing, so waiting too long for amber can result in harvesting over ripe and degraded plants. If you have roughly 95% brown pistils, the plants will be in the ball park for peak THC levels and a mind bending head high. When there is no more yellow pistols and they are all orange then it's also a good indicator its time.
Cannabis plants do feature several other types of trichomes in addition to the three mentioned above, but these are non-glandular trichomes. These are the hair-like or claw-like trichomes which are used for plant defense. And as they have no resin glands, they do not possess the right biological parts to produce cannabinoids.
All glandular trichomes – the bulbous, capitate sessile, and capitate-stalked trichomes – are capable of producing cannabinoids and terpenes. However, they do not all produce cannabinoids to the same extent.
For some reason your plants doesn't want to switch the thc to cbn, this is what would normally make the milky turn amber.
I would harvest based off your pistils, very best to you and good luck.