The answer is melanin. Milady states white hair is the color of keratin without melanin, and specifically clarifies it contains neither type of melanin.
That last clause is the key to this question: eumelanin and pheomelanin are the two types of melanin the book discusses (the ratio between them determines natural hair color), so naming either one alone would be incomplete. The correct, general term the book actually uses is simply melanin — both types absent, not just one.
Milady Standard Barbering, 6th ed., Ch. 18 (Haircoloring and Lightening), p. 644: "White hair is actually the color of keratin without melanin and therefore does not contain either type of melanin."
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