CAS, sorry for spamming your thread, but I wanted to share what I learned about purple stems & purple leaf tips in the last few days. I think generally the purple stem is a sign of N deficiency.... see the attached excerpt from one of Rosenthal's books.
However, I could see how you could get purple stems from over feeding also if you pushed the plant into a lockout at which point it becomes deficient of N and you get purple..... in this case I guess is when you want to flush with plain water. In my case, they were just hungry. Nice new green everywhere.