The answer is pH-balanced. Milady's description matches the question directly: pH-balanced shampoos are mild formulations designed to prevent the stripping of haircolor from the hair.
The book also calls these acid-balanced shampoos, since they're formulated to the natural pH level of hair and skin, usually using citric, lactic, or phosphoric acid. Clarifying shampoos do the opposite job — they're designed to strip buildup — which is why clarifying is a plausible-sounding but incorrect choice here.
Milady Standard Barbering, 6th ed., Ch. 7 (Basics of Chemistry), p. 193: "pH-balanced shampoos, also known as acid-balanced shampoos, are balanced to the pH level of hair and skin... These shampoos are mild formulations designed to prevent the stripping of haircolor from the hair."
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