Fan leafs almost always turn yellow and die off the plant is through with them and uses up the nutrients stored there. It's ok late in flower there is nothing wrong. Only the trichomes will tell you when the proper time to harvest is. The colors of the hairs are only part of the cycle of the plants life. They change color by temperature and maturity at the end of their life cycle. The only true way to know when to harvest is to view the trichomes with a 10x to 30x microscope. Mature trichome looks like a golf ball sitting on golf tee, big and round on the top. The color is clear to cloudy. Cloudy is peak for that trichome. Because there are millions of trichomes you need to harvest on the average. They will continue to mature through the drying process for a day or more. Amber color is past prime for that trichome so you are just looking for an average.