My Trizzlers started to fade at day 72 while I was feeding at ~1.8ec, so I dialed nutes back by half for a week. Then flushed her for the last 6-7 days and the fall colors really started to pop. I should note, I checked the trichs with my microscope before I started the flush and was at 10%clear/80%cloudy/10%amber. So I knew it was getting to where I wanted for optimal harvest. This is an important step, even if you just have a jeweler's loupe. That way you know for sure, but another way is to check the flower's pistils; if they're all golden/brown it's another sign for potential harvest. (Though some autos throw white pistils their whole life.) When I start to see golden pistils I know we're getting close.
I've also heard it isn't necessary, so it's up to the individual. I like to save a couple bucks if I can, so I "flush" at the end. (7-10days before harvest)