This is a sign of phosphorus deficiency! It might be due to incorrect substrate pH. Or maybe you're overfeeding it with nitrogen and forgot to add phosphorus! If you're growing in soil and not using any fertilizers yet, it's likely that the soil contained little phosphorus and needs a Bloom supplement added to your watering! If you're already using fertilizers, then reduce the Grow supplement and increase the Bloom supplement! 👆 The ratio for the middle and end of the vegetative stage is 1:1 Grow:Bloom 👆 Good luck, bro! ✊😇