Could be a couple of things wrong there but I agree with Chubby its a deficiency not a toxicity so I would up the nutes as Chubb suggested. I only grow in hydro, water or coco, but I always run nutes at 100% until I see a problem but soil is different and I am just learning how to grow dirty. In veg a little foliar spray with dissolved Epsom Salt and/or Cal/Mag will darken them up overnight. Spray the bottoms of the leaves though