I would wait a week or so before thinking about feeding. It is young and probably damaged roots so dont take any risk to burn the rest of them . Also im not familiar with that brand.
Yes, it can be a shortage or a block from something else. Finding what causes the purple stems is as important as fixing it.
Good starting point about basic plant chemistry are videos from npk University, lectures from Harvey Smith.
Yes i do trade in some weight for taste but i grow for own use so that isnt that important for me.
At the moment im in a Indica phase but a nice high sativa is always welcome. Normally i have multpile strains going (sativa and indica) but not this time. Its round two of the melon gum...
Personally i think when your high/stoned isnt what you want that you have to change strain. Milky trichomes are at their peak and give all treats from the strain. Thats my preference and theory behind it, but to be honest i cant scientifically proof it :D