I admit, i know little to nothing about this dude, but from what i've read, which obviously could be wrong, it could be down to nutes, heat, soil or water ph, humidity or even just the plants maturing... so pic it up and im sure someone can fully help, personally i wouldnt put it on the heat though, ive been running my grow at 80-90degrees and they are a lovely green colour, had a few light yellow leaves appearing but they were just a bit thirsty it seemed. and on the recycled part, recycled from previous grow? seen sometimes if soil isnt recycled properly it could affect it, and if YOU recycled it from a previous grow, is it gonna have any nutrients in the soil at all? could be it, just flinging ideas and probably aint helping much lol
are you feeding them and from when?