So asks a commenter below in relation to the question above. First, why would one even presume that they were red-haired, see my 2007 post, or the paper in Science: A Melanocortin 1 Receptor Allele Suggests Varying Pigmentation Among Neanderthals. In humans loss of pigmentation can usually be thought of as loss of function on genes. That’s probably one reason that there are several different genetic architectures for light skin, but only one for very dark skin. There might be only one way for an engine to operate as designed, but there are many different ways you can tweak a part and render it broken.
The reason that scientists have posited that Neanderthals were red-haired is that they examined the region around a melanocortin receptor gene which serves as something of a master regulator in terms of melanin production. Their sample was of two Neanderthals, one from Italy and one from Spain, and both exhibited signs of loss of function within this region. An N = 2 is small, but one must recall the fact that they are independent draws as they were sampled from different regions. Also, since then from what I …