I have seen things like this turn up in fic before and it is sometimes baffling. I once saw Adrienne Palicki from Supernatural described as a petite brunette WTF.

I don't see how Anakin, unless he's a child, be described as a blonde. It is light to medium brown as an adult, although depending on lighting it can almost look blond given it's very short in Attack of the Clones, yet there are photos I've seen from the same film where it looks dark brown. In short, brown hair of any tone can look distinctly lighter or darker depending on lighting, whereas a bright blond is almost always clear as is a deep brunette.

I can, however, see why Obi Wan is called a red head because in Clone Wars he sports reddish brown hair. My guess is that this choice was made in the series to help distinguish him from Anakin. In the movies, he is blondish. I have two cousins who were fiery redheads as children and both faded to a strawberry blonde in later adulthood. The real question to me is what other features mark them as redheads. I don't see from skin tone or freckling that Obi Wan is a natural redhead, and a man's beard can show up differently from hair color if they are, for example, a faded blond or redhead.