How I handle green-toned ends when doing a root touch-up

When I first took over the salon and started doing cool brown touch-ups for clients, I encountered an easily underestimated problem: clients who had previously used box dye at home would have ends that faded to a greenish-gray tone, and applying the target color directly would just make it look muddy. It wasn't the kind of failure where the cause is obvious at a glance; it was more like several…