Update: tried a 10 volume developer on a level 9 blonde and her scalp turned orange for 3 days
Did a root touch up in my chair off 5th street last Tuesday and used 10 vol instead of 20 to be gentler since she said her scalp burned at her old salon. Next morning she sends me a pic, her part line is straight up tangerine and she swears it was not there before. Turned out the cheap 10 vol I grabbed was actually labeled wrong or old or something, it lifted way harder than it should have. Now I test every new bottle on a strand from my own head before it touches a paying head, anyone else get burned by a mislabeled developer?
A mislabeled bottle wouldnt cause the scalp itself to go orange though. Developer doesnt deposit color, it only opens the cuticle so whatever dye is mixed in can get in. If her part line was tangerine the next day, that sounds more like the color itself oxidized or grabbed unevenly at the roots, maybe from leftover buildup or her scalp being sensitive from the last salon. The 10 vol thing matters but it would make the lift weaker, not stronger, unless the bottle was actually 30 or 40 vol and just marked wrong.