Seeking help - leaves turned black / dark purple

Hey everyone, first of all, I have to say - I've learned a lot here ever since I've started lurking about, but obviously not quite enough yet, as I am in the flowering stage of my first grow and have encountered an issue and am hoping some of the great people I've seen around here could help me!

So, to start off - the plant in question is an autoflowering Sweet Cheese and it stayed quite short due to some soil issues in early veg state. It's now around a foot and a half tall and is in a 3 gallon container. It's being grown under a 250W cool-tubed HPS, which I had to move quite far away (about 50 cm), as it is in the room with a much taller Northern Lights plant. Ever since I had to move the light up, I put another 24W CFL right on top of it so she gets a bit more light to help her out. The temperatures in the grow room are around 25-26°C (77 F) during lights on, and around 18-19°C (65 F) during lights off. The humidity varies from around 40-60%.

The soil I am using is just some regular potting soil with added nutrients which are all probably long gone, so I've been using HESI's Bloom Complex (4-3-5). I usually put around 25 ml in around 5 litres of water (as per bottle instructions), which brings the water/fert solution to around 550-650 PPM. The water I'm using is sadly quite high with PPM to begin with - around 270. I'll usually feed every second watering or so, and I water once I feel the pot getting light, which is about once a week.

The problem started occuring maybe 30 days into flowering and has spread all over the plant by now (in a matter of 15-20 days). She's pretty much almost done flowering by now, but I would still like to resolve the issue, as I'm starting to see a similar pattern on my other plant, which too just about entered it's 30th day of flowering .

Anyhow, the plant was perfectly healthy until a couple of days into the flowering stage I started noticing a lot of the leaves turning yellow. That's when I decided to use a bit stronger nute solution, but not much. I continued doing so and started seeing an improvement, when all of a sudden, I then started noticing some leaves going dark purple / black. In fear of it being some kind of nute burn, I watered with plain water for 2 weeks, but the leaves kept on going darker and darker, until all of the plant started looking like the picture below. The buds otherwise look healthy and thick, the trichs are now mostly cloudy with some already turning amber as well. Anyhow, after not fertilizing didn't appear to help, I started fertilizing again without much success. Now I'm kind of stuck and not sure what to do further. I'm still thinking I might've over-fertilized it due to the burned tips, but not using any ferts for a while didn't seem to do anything either. Maybe I should've flushed?

If anyone could shed any more light on the issue, it would be much appreciated, since like I've mentioned before, the same thing seems to be happening to my other plant .

Sorry for the wall of text and thanks to anyone who'll take their time to answer!

And here's the pic:

Pic of whole plant might help. I don't know off the top of my head. Some plants are known to go purple and black during late flowering but since you have two different strains doing it, it may be something else. The only things that come to mind from your details are maybe a spectrum issue with your cfl (total shot in the dark) or an issue with your source water. Do you know what is making up those 270ppm? Can you get a water report on your supply?

It's Normal. That's it's Pheno color

Thank you both for your answers!

@Bugeye - I'll get a pic of the whole plant tomorrow, they're asleep atm. Spectrum range with my CFL could be an issue, since it's 2700K - I've read many times that it shouldn't be a problem, so I just decided to use it. I'll try turning it off though. As far as the 270 PPM in my water - I do have a water report, but I'm not really sure what I should be looking at, as there are so many elements listed on the chart it's quite confusing, although I can't find anything about iron content or something alike. It's mostly pesticides and metabolites which are mentioned. It doesn't help I have no clue about chemistry ;p. I do think the pH might be slightly higher than the plants would perhaps like, but as I said - I haven't been able to afford a pH tester and corrector yet, nor do I know if pH of perhaps 7-7,5 could cause this issue? And if it could, wouldn't I see some problems during the vegetative state as well?

@Sp33dymonk3y - I started thinking the same, until a completely different strain started doing the same as well, so I'm not completely sure that's the reason . I wish though ;p.

Oh, I just remembered - my fertilizer bottle says that it will slightly lower the pH value of the water once added. Is it possible that it's turning the water too acidic? Or are these fertilizers usually not that strong when it comes to altering the water's pH value? Each day I seem to find more and more reasons to really try and get a pH tester. Think I'll go get the paper indicators to start with, just to have a slight idea of what I'm dealing with.

Wow this is a tough question.... I get crazy colors a lot, and it usually results in the bottom leaves dying. I lost 14 plants due to this shit. I fixed it by adding some lime to my water which evens out the PH. It helps a lot. I still get yellow leaves but the plants dont seem to mind it. Dont go crazy over this, it is really frustrating at times. Try adding lime to one, plain water to another, and see what happens.

Haha well it could both different strain of plants have black purple pheno color?(;

Heya, as promised, here's a picture of the whole plant. Had to use flash so it looks a bit washed out. Other than that, I haven't got any additional info yet, I'll be grabbing some pH indicators in about an hour though . Any kind of help / ideas are still greatly appreciated, thank you!

That late into flowering your fan leaf color really doesn't matter. Enjoy your buds, they look nice!

Purple Kush makes leafs like that towards the end of flowering. Purple Kush starts flowering with dense green buds and finishs with dark purple dense buds.

More then likely its genetic and nothing you can do about it.

Thank you for all your answers! The plant has now already been harvested, but after a little more research, I am pretty sure it was a case of phosphorus deficiency, which would make a lot of sense, as up until yesterday, I did not know that HESI recommends adding another solution of their own called "Phosphorus plus" along with their standard bloom fertilizer once the plant reaches week 3-4 of flowering, which is exactly when both my plants started having these issues. I'll now be adding this to my water and hopefully won't have any problems with it again!

Again, thank you for all your help !