Side note

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Those first two (rounded) "leaves" that have gone yellow are actually called cotyledons. Their only purpose is to protect the growing tip as it pushes through the soil. For them to turn yellow and die in the first few days is perfectly normal and of no consequence, they have nothing to do with the growth or health of the plant. 18/6 is much more cannabis friendly by the way, cannabis needs a good sleep every day to function properly. No need to feed the plant just yet, there should be enough goodness in the soil for the first couple of weeks growth, providing it was decent soil to start with. High amounts of perlite are qite unnecessary, it only reduces the amount of available nutrients in the soil mix. Most modern soil mixes have good drainage and porosity, making the addition of large amounts of perlite unnecessary, it really is only needed for poor draining soils and if used in with modern soil mixes, really only needs to be 1-3% at most. Anyway, your plant is fine and the cotyledons turning yellow. dying and falling off is all part of normal cannabis life cycle, so no need to worry. Hope this helps, Organoman.

That seed just popped give her a bit of time before feeding if you are in soil. If it's in soil and the soil has any fortification, it should not need feed for at least two-three weeks. If it is not soil and its soilless with not fortification you will need to be using very week feedings. like 1/4 the low amount to feed your plants. No idea of your feeding schedule or what you intend to use. If it dry organic amendments, get a PH meter. If its liquid nutrients, get a PH meter and a TDS pen. you will need both tools to calibrate the feed solution to the desired EC and PH. Best Of Luck!

The yellowing is most likely causes by too little nitrogen available to her, I'd start with a really low dose of mutes, no more than quarter strength to start. Sometimes transplanting can stunt growth a little. Give her time and she'll bounce back.