The trichomes don't have to be amber but imo only the trichomes are going to tell you when it's ready. And "ready" is a personal preference. Take a small bud off when the trichomes are cloudy, smoke that, is that effect what you prefer? If not, wait, as you see trichomes become amber try another bud. In the morning I want an "early" bud, a lifting effect (where the trichomes were cloudy but not amber). At night I'll probably prefer an amber harvest for the couch lock effect.