legolas's hair

[Galin]

For example, it's often said (with citations) that ALL Noldor (save Finarfin's descendants) are dark-haired; yet we learn later that Feanor's wife Nerdanel and at least two of her sons were redheads.

If I recall correctly Nerdanel herself is noted as having brown hair and a ruddy complexion. As already noted, Maedros, Amros and Amros have the rare red-brown hair of Nerdanel's kin (using the forms of the names that go with this notion).

Just to note it: the Appendix F description (dark-haired save for the House of Finarfin) actually concerns the Eldar, not merely the Noldor, but I fully agree that this is a general description in any case, allowing for exceptions.

And I don't think we necessarily need any of the Silvan Elves to be Avari to be golden haired, as according to The Lord of the Rings at least, the Silvan Elves of Lorien and Mirkwood are not considered Eldar in any case.