Interesting how this is so different across the globe. I noticed in the US people tend to harvest when there is just a minimal amount of amber, basically trying to avoid amber and max milky. In the Netherlands 30% amber is common, in Portugal and Spain they wait even longer.
I've seen the post and pics with the "scientific proof" someone referenced earlier in this thread and I have a hard time believing it. Milky is THC and amber is degraded into CBD? Sounds too simple. I recently read a review of some strains in Portugal or Spain that had CBD percentages nearly as high as we have the THC (matching their late harvest habbit). However,while 30% amber is common here in the Netherlands, the THC % of what we call Netherweed has significantly increased over the passed decade while the CBD % decreased.
Since recently we have the Amsterdam Test Lab, which does highly accurate tests on mj, resulting in a list with substances and their %. Costs about $100 for a test (more if you want a certificate) but can test multiple samples in one test. Just started a new round, but will put it to the test in about 3 months. That is, I will have the THC and CBD compared between two buds from the same plant, of which one is harvested early, and the other is harvested late.
I like to see a minimal amount of amber on sugar leaves and the occassional amber speck on callyxes, which here means I like my weed high. Any sooner seems personally a waste of good weed (they fatten most near the end). Edit: I'd be more worried about clear than amber.
Usually the plants reduce their drinking a lot when they are closer to harvest, then I give them even less to force the fall to set in, then wait till the chlorophyll got sucked out of the leaves and yellowed a bit, then 2 days no water and no light, then harvest. I can imagine that without the induced fall and sufficient water the plant will continue too long resulting in more amber.