well, the finger thing probably isn't a good idea in a solo cup. it's better to pick up the pot and see how light it is. make a point to feel the weight of it when you're putting soil (or coco or promix or w/e) in it dry. you don't want it to get that light, obviously, but closer than you might think.
it is better to underwater than over water, but it's best to water the right amount. that depends on what kind of medium you're growing in, how vigorous the plant is, what the temps, RH and airflow in the room are.
as far as the purple stem, it say its mostly genetic, but cold roots (from being wet all the time) could be contributing to it.
didn't think to mention this earlier, either. if you have a fan in your tent, don't have it blowing directly on your plant. its much better to aim it at the wall behind the plants and let the backwash hit them.