i mean, the number one killer of young plants is OVERWATERING, and the number two killer is Nutrient Toxicity.
do not just start doing shit willy nilly, water only when youre certain the moisture in the soil is low, and allow for good drainage. if you use a pencil or a cooking skewer you can puncha shitload of holes in the sides of the solo drinks cup all the way up, dozens of them, and this will help your plant grow, both above and below the soil. roots need air, not just water
miracle gro soil wil NOT need nutrients until youre on the third or 4th node in a solocup, and it's well past time to move them to a much larger pot.
if youre growijng in a solo cup you shouldnt even be thinking about nutrients unless your using strerile nutrient free products like Sunshine Mix, rockwool or coco and perlite. youll be root bound and dead from constriction long before the plant runs out of nutrients in a solo cup.
do NOT add fertilizers, do NOT over water, and do NOT freak out. just wait.
stems can be any of a number of cours and thos colours can change many times. stem colouration means very little as long as it's not Black. Black is always bad. or stark white, stark white is also always bad except for roots and pisitils .
your plants can look a little sickly when growing roots instead of foliage because they should be able to do both at the same time. your plant may need more light (cant tell, no pics) to support proper foliage growth and proper root growth, but when the plant must choose, it will usually grow roots first, and then start growing foliage when the root system is established. the reason is obvious, a plant that has too much foliage and not enough root falls over in the wind and dies, but a plant with more root than foliage is "well established" and can begin growing foiage at it's leisure.
