"fall colors" at end of grow... mostly genetics or other causes?

Lights have been on 19/5, during which the temps have been in the low 80's. 'Lights off' the temps never go below 68º, so I really don't think low temps are the cause. I did mistakenly put my light at 400w when I meant to have it at 300w, and it was like that for a long time before I caught it, like most of the flowering part of the grow. It never bleached the top leaves the way too much light can do, but I don't know if that had any impact on leaves turning. After discovering it, I turned it down and raised it a bit.

During the last week and for the rest of the run I kicked the fan up a notch, moving the temps down to around 76º. Even if the colors might look pretty, the plants look stressed to me. I think over the next couple of weeks I'll be at least harvesting the tops. Since it's a dense run and since I have one Dutch Passion plant that has many weeks left to go, I'll probably do a staggered harvest and cut the main buds while the lower parts of the plants get some light and mature some more.

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If you have a 15 degree temp swing lights on/lights off that is definitely what's bringing out your colors. Low temps, temp swings all contribute the plants turning purple. In addition to genetics. Every single auto I've grown has shown purple somewhere because I have a 15 degree day/night swing. I don't mind it, purple bud is pretty IMO.
 
This is the most dramatic one. But she sure is pretty. The guy I got the beans from has never seen one of these GSC turn purple. It’s my temp swings.

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I appreciate all of the comments, but there are no cold temps in this grow, and purple is not the primary color, yellow/red/orange is more the spectrum. The exception was Purple Rolex, which actually did turn purple. It's all good, the buds are finishing well, so if I never come to understand what's causing it, I'll just enjoy the show.
 
There doesn’t have to be cold temps. Temp swings will cause it as well. My temps don’t go below 70.
 
Another plant. This happened within the last 24 hours. Every single auto I’ve grown has had some purple. Temp swings, I’m telling you.

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