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@cyberia If you think about what you said, their combined effect actually makes sense. Ammonium hydroxide is an effective hair bleaching agent, but it can't just be used since it's a powder. Mix it with hydrogen peroxide cream, and it's now dissolved in a way that allows it to actually be used. And, well, the hydrogen peroxide, though not very effective, might still pick off anything the ammonium hydroxide missed, which might make it more effective than a pure ammonium hydroxide cream.
@cyberia As to the latter bit, I would guess it's just the hydrogen peroxide breaking down into water and oxygen gas due to light exposure. At least, I'm not aware of any ammonia/hydrogen peroxide reactions that might otherwise be happening, and I can't imagine anything in it is otherwise reacting to the air around it. And since the bottle for the hydrogen peroxide is opaque, that would massively slow down the rate at which the peroxide breaks down.