I don't know about the tipping, but I actually would be bothered by a salon that had a trainee fix a botched job by a stylist. IMHO it says the salon owner/manager isn't really concerned with the customer's satisfaction ("oh, you don't like it? Here's someone brand new that we're training; we'll let them try to correct what the trained full-fledge stylist did wrong.")
I would also think the salon wouldn't let you pay anything, tip or otherwise, for this visit. However, if the trainee did an awesome job, I imagine I would tip them out of gratitude.
I hope it came out better the second time around. Did you end up tipping the trainee?