Complexion can mean characteristics of other things too. It does seem pretty clear in the statement but I decided to give him benefit of the doubt.
Why I asked about the parentage. Are there dark skinned elves in the Witcher lore? That's the next question. I can't think of any. Nilfgaardians are white.
The definition of the word "complexion" is pretty clear in that setting, even most settings.
There literally is no other way of interpreting that sentence. Unless one suggests that the character who observed Ciri in that scene could somehow see her emotional nature or her body's complexities involving liquids, heat and other measures.
Asking where it says that someone is white, based on the description "ashen-white complexion" is like asking how a car is blue based on "the azure paint reflected the sun".
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