In the book, Legolas' hair color is never specified, but J.R.R. Tolkien did detail certain rules for which Elves have which hair colors since all elves are descended from two lines: the Noldor, who have dark hair, and the Teleri, who have either dark or silver hair. As a Sindar Elf, descended from the Teleri, it seems that Legolas should have either dark or silver hair.