I am playing around with LED's this year as well. I had the same issue as you with all of my tomatoes. Purple underside of leaves. I did fert but it didn't help. I think it may have something to do with the strength of the LED's. I have a 45 watt and a 300 watt. The tomatoes were under the 300 and I had it raised about 10-12 inches above. They were not growing much, in fact some of the leaves started to dry out even though the soil was moist (not wet). Now I am not sure if they just out grew the purple leaves or what I did next was the reason. I raised my 300 watt an extra 12 inches. Soon as I did that, the tomatoes really started thriving. Nice thick stems and root system. So many sights say put the LED just a few inches above them. I'm not so sure about that. I saw one video which demonstrated that too strong of an led light bombarded the plant in so much photons that it stunted the growth. Hope this helps, and I would not worry so much about the purple leaves in your seedlings.
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