Oxymelanin
A: Oxymelanin is the byproduct of the bleaching process that takes place when you perform hair color removal services of any type (even the naturally-occurring bleaching of the hair when in the sun).
What actually occurs is that the hydrogen peroxide oxidizes the melanin of natural pigments (melanin) and forms a colorless substance called "oxymelanin".
The oxidation reaction is caused by the alkali in the hair bleach mixture reacting with the hydrogen peroxide. It causes the peroxide to release nascent oxygen which is what causes the release of hair color, and leaves behind the pigment molecules in an altered form, which is where the "oxy-melanin" term comes from.
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See also:
Melanin and hair color
The bleaching process
Where does the color go when you bleach dark hair?
How often can you bleach your hair?