Ok so I've been shown the chart that helps pick. I think it looks like N or iron. I went with iron because I'm only 2 weeks in flower and they got grow big last week still. It's mainly on middle leaves so I put it down to being seeded but there not yellow like the ones I pull of at the very bottom. I'm a little more worried because the top two plants look lighter green. The bottom left hardly any signs whatsoever. All on the same feed. I added cal mag in there water for the first time 3 days ago. The stems have always been slightly purple but notice on some expecially ones looking this colour are extra purple. Also in the ones I picked off, when I tried to use my nail to pinch them the stem was to tough and I couldn't. I had to use the pruners. The ones at the bottom that yellow due to no light normally just fall of when I brush them. I'm asking here because I would like some expert tell me exactly what it is if possible. 12 weeks in not a problem so far, really do t want it to go start slipping up now.