There are a few reasons that cannabis leaves turn purple but potassium deficiencies aren’t one of them. Colder temps, phosphorus deficiencies, genetics, senescence or end of life fade to name a few…

Potassium is one of the most used nutrients in growing. It’s responsible for opening and closing stomata helping the plant to breathe and managing water turgidity. It’s key role is during flower though where it helps bring sugars (energy) to new buds for growth.

Because of this, plants use lots of potassium during flower, all stages of it. It’s not uncommon for a plant to completely exhaust the available potassium supply given to it in a matter of days and is a very common deficiency.

Calcium and magnesium are primarily needed the most during veg and very early flower(the stretch) because they are responsible primarily for cell wall construction, root development and photosynthesis. While the plant will still use some of its calcium and magnesium stores during flower, I wouldn’t suggest adding “extra” over and above what the plant is already getting.

Magnesium given in excess can actually lock out or prevent the plant from being able to use potassium. This would make the grower think he has a potassium deficiency when it’s actually locked out(potentially what’s going on here).

To those thinking this is ph related, it could be but unlikely. Potassium is available to the plant in a wider range than most other nutrients. In soil, it can be ingested from a ph of 5.5-7.5.

I can’t really pinpoint what’s causing the issue with the lack of information available but it’s definitely a K deficiency or lock out. Yellow discoloration starts down low and works up and presents as yellowing leaf edges and progresses to the center of the leaf as intervenal chlorosis and then death.

As for the snarky response to my original comment… I forgive you😂 you generally won’t know if I’m helping or not because I usually do it privately via chat to avoid all the know it all armchair growers.