Instructor at my Beaumont certification class swore by a 20 volume developer for grey coverage, cost me a client
Did a full grey retouch on a client's roots with 20 volume like she drilled into us, and her resistant grey came out orange, so I switched to 30 volume for grey retouches after that and never had the problem again, has anyone else had a trainer give them advice that flopped in the real chair?
Tbh I always trusted the 20 volume thing too until I watched resistant grey stay stubborn on a few clients. Sounds like your instructor's rule works fine on normal hair but falls apart on the tough stuff.