It's traditional to make it gold. LOTR artists have been doing it since the books came out, likely because, to their minds, Elves have fair hair. However, it never actually says what the colour of his hair is, which is what left open the debate. What answers it with 99% surety is the Silimarillion. You see, Thranduil of Mirkwood, Legolas' father, is almost surely a Grey Elf... just checked and the Silmarillion says he's a Sindarin, ie. Grey Elf. This means beyond doubt that he is one of the Teleri. Now, there were three major groups of Elves: Vanyar, Noldor, and Teleri. Of these, only the Vanyar had golden hair, and all of these went West 20,000 years before Lord of the Rings. The reason some like Galadriel have golden hair, even though she is a Noldorin Elf, is that her father, a prince of the Noldor, had Indis of the Vanyar as a mother. But that's a rare thing, and only a few like Glorfindel and she have such light hair. But that's the Noldor; they were in the West with the Vanyar. The Sindarin Grey Elves, a subgroup of the Teleri, remained in the East. Thus unless some gold-haired children were born to them before this sundering, or unless Thranduil or his kin married into a golden haired house (both greatly unlikely), Legolas must have dark hair, as a Sindarin Grey Elf.
