In Lord of the Rings, Galadriel’s hair is described only as a “deep gold.” But, as Tolkien developed her character further, her hair became an increasingly complex - and significant - topic. Her gift of hair to Gimli, and her refusal of the same give millennia earlier to Feanor, make Galadriel’s hair a surprisingly important part of her character development. (If interested in these stories, I’d recommend you read this post.)
So, it’s in Tolkien’s later writings about Galadriel, when he fully realized the importance of her hair, that we get the best description of it. In “The History of Galadriel and Celeborn”, it’s described as such:
Even among the Eldar she was accounted beautiful, and her hair was held a marvel unmatched. It was golden like the hair of her father and of her foremother Indis, but richer and more radiant, for its gold was touched by some memory of the starlike silver of her mother; and the Eldar said that the light of the Two Trees, Laurelin and Telperion, had been snared in her tresses. Many thought that this saying first gave to Fëanor the thought of imprisoning and blending the light of the Trees that later took shape in his hands as the Silmarils.
So, basically, it was a sort of mix of gold and silver. Both representing her Vanyarin and Telerin heritage, but also representing the mixed gold and silver light of the Two Trees of Valinor.
SOURCES: LOTR, The Unfinished Tales (“The History of Galadriel and Celeborn”)