Actually, the color amber is a dark yellow, and I've never seen any pistils turn to that color. The pistils are AKA "white hairs," and when they wither, they usually turn a brown or dark red color, and are then known as "red hairs."
So frog is right about that, when 80% of the white hairs have turned.
The amber is the color that the resin becomes when it is past ripe and starting to degrade. The resin is inside the trichomes, which are AKA little mushroom-shaped glands.
HTH