Not a word is said in the Silmarillion on Fingolfin’s hair colour. Finarfin is expressly said to be blonde, which causes many people to believe Fingolfin was too, but that’s an error, plain and simple. Cf. the appendix of the Silmarillion under Finarfin: “Alone among the Noldorin princes he and his descendants had golden hair, derived from his mother Indis”, p. 330 in my Harper/Collins edition. Descendants, not siblings.
There is one single passage in the published Silmarillion which seems to have caused this mess, and that is that “Fingolfin and Finarfin were like Indis in mood and touch”. Incidentally, this also caused me, for a long time, to imagine Fingolfin as golden-haired. But “mood and touch” is not appearance.
Tolkien expressly wrote later (published in the History of Middle-earth, by his son Christopher, not in the published version of the Silmarillion) that Fingolfin, as well as all his children (Fingon, Turgon, Argon [not mentioned in the published Silmarilion either], and Aredhel) had dark hair as well. “Fingolfin was his father’s son, tall, dark, and proud…” (The History of Middle-earth volume 12: The Peoples of Middle-earth. Edited by Christopher Tolkien, HarperCollins Publishers, 1996. Page 336 in a text called “The Shibboleth of Fëanor”, written by J. R. R. Tolkien in or slightly after 1968.
So, even wonderful and accomplished artists like John Howe and Ted Nasmith got it wrong when they portrayed Fingolfin as golden-haired. Not that it matters, honestly. To depict a character whose hair colour is never mentioned in the book with the wrong hair colour is a lot less bad than what Iron Crown Enterprises did with Lúthien, describing her as “having the beauty of angels, with golden hair and sparkling blue eyes” in an RPG supplement.
(By the way, if you insist on getting nerdy on me: These are the two most ridiculous comments in that regard that you’d have to strive for.)
1) “A skilled artist thou art, though the tale wit thee (!) not well; Fingolfin’s mother, Indis the Fair, of the Vanyar was, and as her sons both, golden haired. And quoth I from the pages of the Quenta Silmarillion: […] Wit I well the lore ancient, and speaketh (!) I the truth; many an hour have I spent wandering o’er the pages of volumes long past writ. Learn thee (!) the tales proper before again thou dost dare of them to speak.”
Needless to say, the Early Modern English there is just brimming with errors.
2) “Yes, I am elenwe the lost wife of turgon, and i have returned to say that my father-in-law had blonde hair”
The strangest thing about those obsessed Blonde-Fingolfin-groupies is that they claim to be the staunchest Tolkien fans, and rave about how I’m besmirching his memory with my lack of research – when Tolkien himself was known to be extremely liberal with people’s visions of his characters. That’s why he barely depicted the characters, only the locations, in his own illustrations. That’s why, in the first editions of the books, he wanted no illustrations prescribing readers how to imagine them. If it had been important to him how exactly we ought to see Fingolfin, he definitely would have described him in more detail.