the leaves of my plants are starting to turn purpl...

As you can see in the photo, the plant is nearing the end of its life cycle and the leaves are turning purple because some strains are genetically like this, other and residual reasons for the purple color may be wrong PH causing a phosphorus deficiency in the plant at which point the leaves turn purple but your plant does not have a similar problem in my opinion. I wish you success

With the little info you gave, you are showing some need for nutrients, but I have a feeling that you are nearing the end of your cycle and the plants are going through the same changes that happen to leaves on plants in the fall. They finish their life cycle and as you lose nitrogen the plants colour begins to fade and the leaf will ultimately die. Happy Gardening Growmie. 👊🇨🇦

Don't even know what you're growing here.... no diary = no information. It could just be the strain as there are a lot of purple strains out there... the only factor that YOU can control that would have the leaves turning purple is your pH is way too low.... if you maintain a pH between 6.0 and 6.5, then it's just the strain but considering the yellow color in the other leaves, you've got some nute/pH issues you haven't addressed... Good luck