What is the issue with this coloring?

Nitrogen toxicity (as evidenced by the super dark green leaves) from feeding too much or too often.........or both. Remember, in soil it should be a schedule of plain water/plain water/feed/plain water/plain water/feed.........so that your plants are only being fed every 10-12 days or so. Also, you should really consider going to 18/6.......EVERYTHING on planet Earth NEEDS sleep to function properly, even autoflowering cannabis! Without a dark period every 24 hours, your plants can not finalize their photosynthesis cycle and therefore have a correctly functioning metabolism. A plant with an incorrectly functioning metabolism will never be able to grow to its fullest and healthiest potential with cannabinoid synthesis and trichome production suffering. The MYTH that it is ok to grow cannabis for weeks/months at 24/0 without sleep, needs to be removed from the cannabis growing community ASAP!

its fine Watermelon is known to produce a purple pheno seems like you will get yourself some purple leaves/bud enjoy 😄

Light nitrogen excess and magnesium issue. Could be related to too much Nitrogen, that could influence absorbtion of magnesium. Could be wrong ph. Could be wrong watering regime, when you dont yave proper wet/dry cycle. Could be just LED lights, plant need extra calmag feeds under them even in soil . Could be just hungry for magnesium. Think thats all bad possibilities, sort yours ! I would go 2-3 cycles only with right ph water no additives.