Seedling leaves turning yellow coco

Those rounded growths known as "cotyledons" are not leaves in the true sense and for them to die quite early is perfectly normal and not a sign of any issues or problems. Their function is to protect the meristem/growing tip from damage as it pushes through the soil to the surface, where once there, the cotyledons fold back to expose the first true leaves, the familiar serrated edged leaves. Hope this helps,............ Organoman. P.S. - in my experience, those cups are not that friendly for roots to "escape" from, so when transplanting, it can help to make a few slits/slots/cuts, very carefully, for the roots to go forth and explore. Those type of cups seem to need a LOT of moisture to breakdown and work as they are advertised, and that is something cannabis is not keen on, wet feet. They also have an ability to literally "suck" the moisture out of the substrate, meaning watering often, which then leaches the soil/substrate quickly of any goodness. They are also a bit small - 4 inch wide by 5 inch deep black plastic pots seem ideal, and if cared for and washed between uses, last many years, making them quite economical in the long run. Cheers.