The answer is ash. A cool, gray-toned cast showing up unwanted is the sign hair's been pushed past a safe lightening point.
Gold, violet, and red are all tones you'll see at different, earlier stages of the lifting process — they're not the warning sign for going too far. Ash specifically shows up once hair has lost enough warm pigment that it's sitting in risky, over-processed territory, which is why it's the tone to watch for and correct before damage sets in.
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