Looks really dark green, and that should mean too much nitrogen, water and not enough magnesium. I have a white widow that stays really dark too. The curling usually means something wrong in the root zone. What are your ppms, in and out? Do you notice bug evidence? Are you giving them enough time to dry between waterings? Is this Hydro? What strain. Some strains do not like water too much towards the end of flower, but it doesn't look like you're there yet. Back off of the watering and get some epsom salts in there.