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It depends what week its in.. some strains dont show much amber at all - some turn purple instead ( freaked me out the first time I saw it). As long as the trichs are cloudy/milky, its been in flower for 9-10 weeks and the plant looks mature - I harvest.

I've consistently harvested each new strain I run at 5%, 10% and 15% amber. I've found less amber to cause reduced time of effect. More amber starts to get a sleepy effect, regardless of the strain the last 15 years.

I'd be interested in hearing the results from your own tests, the same way. No amber means its a very long flowering strain/pheno, haven't seen purple but I'd treat it as amber if I did.

Cool beans. ;)