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. ;)