leaf colour

Hey Minor Monster. This looks like a magnesium deficiency to me, and it happens quickly under LED during flowering. Often it comes with calcium deficiency if you discover rusty spots on the upper leaves. It could also be related to the pH level; both are absorbed most efficiently at the upper end depending on the medium (soil pH: 7, hydro pH 6.0-6.2). The reason it's a common phenomenon under LED light is the leaf temperature = less transpiration; the transport from roots to tips is a big effort for the plants. Magnesium is a mobile nutrient, which is why the deficiency symptoms show up first on the large leaves at the bottom of the plant, which mainly serve as nutrient depots. Calcium is not mobile; it goes directly from the roots to the meristem and cannot be transported from the leaves up to where it's needed. Both are essential building blocks that the plants need primarily for cell formation, which is why it shows up most in the stretch during the first weeks of flowering when the plants are growing vigorously. The first signs of a deficiency are weak leaves and purple stripes on the stems. If there's an imbalance due to too much potassium, it can also occur; both need each other for absorption, and if one is too much or too little, it can lead to problems. So it's best to always fertilize with a balanced nutrient solution. Although almost all growers work with LED today, calmag is never part of the feeding schedules. Calmag is most commonly available as a combined preparation as an additive; I would recommend one with a high nitrogen content, I personally like Sensi calmag quite a bit. I don't know how far along your buds are, but deficiencies can be corrected most quickly with a foliar feed, EC depending on the plant stage 0.3-0.7 pH 6.0-6.3. I personally don't do that anymore at the first flower sites, but there are some who continue foliar feeding until weeks 3-4 of flowering. Otherwise, incorporate it into your feeding schedule, but it always takes a bit longer to address a deficiency through the roots.