Well, we don't know anything about his mother, and we can't be sure she's a Nando. She could have been from
Doriath for all we know.
We know that the
Sindar resembled the Exiled
Noldor in general:
"being dark-haired, strong and tall, but lithe" (JRRT
Quendi And Eldar) -- the description adds that
"grey, pale or silvery grey" does not seem to have been a common feature of the Sindar
"though it was found among them occasionally, especially in the nearer or remoter kin of Elwe (as in the case of Cirdan)."
Indeed it's (Quendi And Eldar again) said that owing to the intermarriage between the
Vanyar, golden hair sometimes later appeared
among the Noldor, notably in the case of
Finarfin and his descendants.
And the Sindar are really a subgroup of the Teleri, as well as the
Nandor. Anyway, I'd ask JRRT why point of that the Vanyar seem the ones responsible for this trait,
if many among the Teleri slash Sindar slash Nandor also had golden hair.
Also, the Sindar are Eldar, and the Eldar were dark-haired save for
the golden house of Finarfin (if admittedly a sweeping comment in Appendix F).
But hold on Elthir!
If "canon" equals
Tolkien-published text (as I believe), then the
Elves of
Mirkwood and
Lorien are
"not Eldar" according to
The Lord of the Rings, and indeed we see a golden-haired
Elf in Lorien that isn't
Galadriel.
Hmm. And if the "Vanyar" are Eldar they would seem to be dark-haired too -- and if so, who knows from whence the golden hair of Finarfin's head springeth and sprouteth?
In the end: my "default colour" for the Sindar, generally speaking, is dark-haired -- based on both Tolkien-published text, and posthumously published Tolkienian description (and in the latter the Nandor east of the
Misty Mountains become "Eldar") -- so
I go with
"dark" unless otherwise described.
My thought is:
Tolkien described the
Elven-king of
The Hobbit without really thinking about it
much. Although that said, JRRT never revised the detail (if he ever considered it later, that is), and so, according to my own rule regarding canon, I must take the golden-haired Elven-king "becoming"
Thranduil father of
Legolas as an in-story truth.
But for me (so far). . .