Based on what I am checking out in the horticulture MMJ bible by Mr. Jorge Cervantes, looks like you may have a potassium or magnesium deficiency. I'm not quite positive. I haven't had too many issues with my leaves turning colors or anything like that. Had a really really small pest problem, but took care of that instantly, and everything is all good now. As for your flowering...I asked about this just the other day. I have been informed that if headroom for the plants is an issue, go ahead and start the flowering cycle, since the plants will double or even triple in height. However, if you have some headroom, go ahead and wait for preflowers to show. Unless they are all feminised, then you should have no problem. I started mine the other day and they were sprouted in mid february. I'm a new grower too...so I'm just passing along info that I recieved. I did just have a thought...I did have some yellowing leaves at the very start of my grow...shrinking...I think it was heat stress, because I moved the light up, opened up the room a little bit to get some nice fresh air in, and they popped right back into shape less than a week later. So check on your lights as well. If anyone agrees or disagrees, let us know. Besides that, everything looks great man. Keep it up.