Hold up, I actually get the struggle. Spent a solid three hours trying to fix a weird green cast on my friend's highlights last month, only to find out her new purple conditioner was leaving a residue. Four hours might feel like a lot, but when you're that deep into a foil and something's off, you chase every possible cause before you check the easy stuff. Normally I'd blame hard water or a bad toner first too. It's annoying but also kind of funny once you finally see it.