I see a couple things that may or may not be the problem. Its most likely a combination of problems. The first thing that comes to mind is that you have them outside where they can get rained on. When they get rained on, a few things can happen. They can get too wet for too long(probably not an issue here given the good drainage). They get all of the nutrients in the soil flushed out and leave the plant with little to eat(I can see this in the lower leaves yellowing off as the plant is eating itself). And the big one: the pH gets thrown way off by the pH of the rain water flushing the soil. I believe the purpling is likely a sign of the pH being far enough off that it is starving the plant of many of the nutrients that it needs.
I highly recommend that you cover the soil when growing outside so that you can control the moisture and pH of your soil. This isn't a problem when growing in ground as the soil cant get flushed as easily (which helps to prevent pH swing).
I would give them a very generous feeding with a solution that has been aerated for 24hrs and then pH adjusted to about 6.3-6.5, and I would set them in pans that are a couple inches deep so that they can sit in the runoff water for a good hour to allow them to soak up any extra water they need.
It is also possible that they have dry spots in the soil from the water moving too fast through it. This can be very damaging to plants when these "dry spots" occur. allowing them to sit in the runoff water for an hour should allow them to draw water in to wet those dry spots. I would also get some dolomitic lime and sprinkle on the soil before watering, and try to work it into the soil rather than letting it sit on top. I would put 1Tbsp of lime per plant and repeat this every 15days.
I don't believe you have too much pearlite in the medium. You may have medium that isn't the best for holding water and/or nutrients but with good medium/soil you can have as much as 50% pearlite without it causing any issues. If you cant find good inexpensive soil to use, I would recommend using coco coir to mix in with some organic soil, or even all coco coir. The coco is excellent at holding just enough water and nutrients, and when mixed with other mediums and/or pearlite, it drains very well.
If you do decide to use coco coir (which I use exclusively) you will need to use dolomitic lime(not calcitic lime) to help give the plants magnesium and calcium which gets locked up some with coco coir. Good luck and green mojo