Fall of Gondolin was written in 1919. Nary a hobbit. Still, The Hobbit is subject to Bilbo's fancy throughout, and can hardly be taken as concrete evidence of anything. That's not to say he would have fabricated Thranduil's hair color. I was a little too limiting in my first comment here -- the golden hair wasn't exclusive to the Vanyar, but it was rare among the other families. Interestingly, it is written that none of the Vanyar abandoned the Journey and became Avari.
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