Don't worry about the purple stems that is normal they look very heathy for now. They do look a little strechy at the moment. How close are your lights? I see you are using T5 lighting, try and get them within 4" of the tops of the plants and raise when needed to maintain that distance.

As for nutes wait until you have your first set of 5 finger leaves, then use FF Grow Big. I had great results from that nute.

Nice setup by the way!