Stems are turning purple near the bottom what can ...

Hello ! It could be a phosphorus deficiency , but I think your plant have serval deficiency ! First a slight nitrogen def as your leaves are already yellowing at week 2 of flowering , you can handle it easily. Second you'v probably have to deal with a slight P def. You feeding chart is quite light for that stage , take a look at the Biobizz feeding schedule to know the best way to feed your plant at that srtage and she'll recover very fast !! Hope it will help you 😁👍