Yo dude,I'm growing ten white widow,4 weeks into flowering and my leaves are starting to turn yellow as well but only the fan leaves and some of the lower growth.I was told that this is normal due to the plant using up a lot of nitrogen during flowering and some flowering nutes don't contain nitrogen. Some of my plants are turning light green as well as some of the lower fan leaves have turned yellow and dropped off.I hope this is all part of the plants life cycle so I've given them a little nitrozyme to boost up the levels,but if it aint,I'm in the shit!!!!