The red hair is true, but simply because the pigment in dark hair nearly always turns red after centuries of burial in certain temperatures and soil chemistry. This is evident in mummies from all over the world, and even evident in ancient Native American scalps. There is no science-based reason to suspect that the Lovelock Culture had red hair; it was almost certainly black, like all native Americans.
The cannibalism is also true, but based only on a very few human bones found at Lovelock Cave that had been split for the removal of their marrow. All others had not. The rarity of such bones there suggests that it was an exceedingly uncommon practice, probably only in times of great famine, and was certainly not the norm.
So where did this idea come from that the Lovelock Culture was a tribe of red-haired cannibalistic giants? Most sources claim it is a Paiute oral tradition. So I did my best to read as much Paiute legend that I could find, having no actual Paiutes on hand to recite oral traditions for me. I found their lore to be speckled with occasional mentions of lone giants in fanciful tales, I found no mention of a tribe called the Si-Te-Cah, a tribe of giants, or any red-haired anybody, cannibalistic or not. The Pyramid Lake Paiute Tribe maintains an active web presence and archives a number of tales, and although you'd think such a prominent urban legend would be mentioned in what they publish, it is not.
What I found, in fact, is that every mention of the Si-Te-Cah appears only in paranormalist books and websites that promote the claim that a Paiute oral tradition says the red-haired giant cannibals were real. Every mention of the Saidaku appears in scholarly books and articles about the Lovelock Culture, with no mention whatsoever of red hair or gigantism. If you're looking for the legend, search for Si-Te-Cah; if you're looking for the true history, search for Saidaku.
