Just wondering just how semi permanent it is... Is it a stain? I am thinking that is what it is... So good old laundry soap will take out hair stain from your hair. If it is more of a tint that does get inside the hair that could be more problematic. To lighten hair you must be able to remove color so that in my opinion is permanent hair color. If the back part is just too light that is an easy fix you can just do the darker color over it. If your bangs are not that bad you could just match the back to the front by buying the one box again. Then try your two tone next time, chalk it up to not being multi-tasker anymore.