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 the book doesn’t tell us anything about Fingolfin. 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.
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 accomplished artists like John Howe and Ted Nasmith got it wrong when they portrayed Fingolfin as golden-haired. Not that it matters, honestly. If you depict a character whose hair colour is never mentioned in the book, it’s not as if it was a terrible crime. 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.