Hello there. I'm about 2-3 weeks away from finish on 2 Autoflower Euforia's I'm growing in 5 gallon smart pots using Super Soil + FF Ocean Forest Soil. Watering using RO water with Cal/Mag added. Light is advanced platinum p300 LED. This is only my second grow ever. First grow had the same problems. Majority of leaves ended up turning almost black and it gave my bud a weird and not appealing smell. Here's a picture of the leaves. From the Euforia.
Worry about the buds not the fan leaves down below. They are supposed to do that during flower stage.
seems like a Phosphorus problem, yes the leaves are supposed to yellow and basically appear as if they are dyeing off, but this level of purpling is a bigger issue. I found a nice article that tells all about it and different methods of treatment you could do. http://hightimes.com/grow/grow-qa-stems-and-leaves-turning-purple/
You need a more p&k heavy bloom booster. It may be too late to help this crop. Feed them more phosphorus and potassium next time.
Get some GH micro and liquid koolbloom. Add 4tsp/gallon micro 6tsp gallon koolbloom for flower phase. Your soil is running out of nutrients at the end of the cycle. You can't raise a healthy to harvest plant on just RO water and cal/mag unless you build a better super soil, top dress with more amendments, feed with more organic teas instead of just water, or use bottled nutrients.
I agree with this, it more than likely is to late, you may be able to help a small bit to insure the buds don't taste TOO terrible, but there will be an off taste at this point, you're to far in with not enough time to completely reverse the damage, but hey, that is ok!!! that is what it's all about, everyone messes up and learns from them, eventually you won't run into many problems, and when you do you will be a pro at figuring it out and solving them. Keep up the work!
Starving a plant a little towards the end can improve the flavor of the bud but if you take it too far it can reduce your yield. It makes for a faster cure and less chlorophyll to eliminate during cure. This is mostly true for nitrogen. It's not ideal to starve a plant of phosphorus or K. Autumn Color Effect I starved a few plants my first few grows.