HELP! Whats going on with my leafs? Purple then black

[kinect360]

Hey!

A few fans of the top leaves of one of my three Purple Haze Feminized plants has started to turn purple then black.. I thought originally the purple might have been because of the strain but I guess not !! Whats going on ??

Feed:
RO Filtered water
Micro Budjuice week 1-7
Nitrogen for weeks 5-7

Switched to 12-12 5 days ago at week 7 because I thought they were autos. They are getting their first feed of phosphorus supplement today.

[UndergroundFarmer]

Did you have a cold night? Sometimes that will trigger color change. On the positive side, it means you got the pheno that actually does turn purple! If you don't mind me asking, what seed company did you get them from?

[kinect360]

Nope don't think I had a particularly cold night, they're in my apartment so the lowest it couldve dropped in the tent might be 17 degrees Celsius. Got them from i49.

The few fans started out turning purple at the tips and then slowly over a week or two, it went from purple, to black. Could it be a phosphorus deficiency?

[UndergroundFarmer]

I think it is either a phosphorus deficiency or something blocking its uptake. It happens to other species too when they don't get enough phosphorus.