Can this be cal/mag?

A couple of pics show some slight signs of a calcium deficiency, but nothing drastic. The yellowing is more to do with "end of life" stage, but it could also be a slight deficiency of either potassium or magnesium, but nothing drastic. The brown/purple colours is just a build of of anthocyanins (plant pigments) and can be due to cool overnight temps, but it is more like how leaves change colour in Fall because they are getting to the end of their usefulness. Nothing needs to be done, there is nothing you can do to "fix" anything before harvest, or the fact that nothing actually needs "fixing". Just continue with whatever you have been doing, and continue with your harvest plans/schedule as normal. So yeah, next grow you can up the calcium a bit, but every plant is different, so what happened with these plants will not necessarily happen with the next crop. You need to watch and react to each plant individually, there is no "one size fits all" solution to growing cannabis plants.