Warning: a client brought her own box dye to my chair in March and I should've just said no
Back in March a regular showed up at my station in Fort Wayne with a $9 box kit from the drugstore and asked me to just apply it. I did it because I didn't want to lose her, and the color grabbed patchy on her porous ends while the roots looked flat. Two days later she called me mad about a green cast and wanted a fix for free, which turned into a 3 hour correction and a $60 tube of toner out of my own pocket. Now I have a flat policy posted at my station but I still feel weird enforcing it, so how do you all turn down box dye without making the client feel dumb?
What if you frame it as the box color being a bad match for her hair, not her being dumb for buying it? Like "this shade fights the warmth in your ends, that's where the green comes from" and then show her a swatch of what you'd use instead. She hears it as you protecting her hair and her money, not scolding her. Then the free fix talk never comes up because she feels like you two picked the color together.