I am a newb, but I have looked into this stuff quite a bit. According to images and charts, it doesn't look like a mg deficiency. Are you feeding them enough? Maybe they are a bit underfed? I am not really seeing many pics that look like yours. They say purple stems are a sign of underfeeding. If the stems are purple, I would try bumping the feed up just a little bit and see what happens.
Again, I'm inexperienced, so take my opinion with a grain of salt. But that's what I would be looking into.