Hi there chro..Firstly wish you positive vibes on your herbal growth, everything looks sweet, but...the purple tips is usually caused by lack of phosphorus, especially in the stage you at now.
as i have experienced a similar deficiency in my nutrients especially in this stage of growth..I know it might seem a bit crazy, But i've been through alot of trials and errors..
This is my opinion, COMPOST TEA IS THE CHAMPION in providing nutrients that are natural and fundamental to creating the perfect soil environment and root development for your girlies.
Plus it's less harsh on your Plant than bottled fertilizer and other store bought fertilizer, NB I am not throwing shade on such fertilizer, whatsoever.
But homemade compost tea (if you need any information on different teas to make for each stage of please feel free to let me know)
Feel free to try this compost tea for phosphorus deficiency...3 cups worm casting, about 2 cups kelp meal, I use molasses about 15ml to ensure the soil is beneficial and mix 20 L reverse osmosis water..you will definitely need a air pump, to boil the tea for 24-36 hours, by than you basically have a super organic system.
Organic tea is less harmful and more beneficial in terms of artifical feeding ..
Feeding one a week