It's an interesting thought but I think the major reason for a lack of discussion is that anime in general doesn't even bring up hair colour as a thing 99% of the time period. Not unless there's a specific artistic reason for it (see raven haired vampires and shit) or a story reason like in bleach (if I recall correctly Ichigo's hair is specifically blonde naturally for some reason) Ultimately hair colour means less than zero, look at evangelion.
Actually, I think that was the whole point of the links I posted. From what I can tell, in general, anime characters are meant to be presumed to have black hair, even if they are drawn otherwise. Obviously, there are exceptions (Saber is almost certainly a genuine blonde, for example), but characters with purple, blue, pink etc. hair are probably just black-haired characters being drawn with artistic licence.
It is, I think, an artefact of the different attitude Japan in general has to theatre. In the West, we tend to try to make plays etc. look realistic, whereas in Japan that was never true. When I went to Japan 3 years ago, we went to watch a Kabuki play, and it came with an English commentary tape. Amongst other things (such as a long description of why an actual
woman couldn't possibly play a female Kabuki character, because they wouldn't "look right"...), it had a discussion of the difference between Western and Japanese theatre, saying that Japanese theatre is more "representational". I.e., they don't try to make things look realistic, they just try to make it clear to the viewer what is supposed
to be happening. As such, they use various visual tricks to get the meaning across, such as giving a character hair colours that represent aspects of their personality.
Another example is that, in Japanese theatre, people dressed in all-black were stage hands, who were there to move props around and, thus, "did not exist" in the eyes of the audience. This is, in fact, where we get the classic all-black "Ninja" costume from (since, in reality, Ninjas, as assassins and spies, had no formal costume and would simply dress in whatever allowed them to blend-in the best, which would sometimes include a navy blue outfit for night operations, but not really pure black). At some point, some clever writer realised that, since the stage hands were presumed not to exist by the audience, it would be genuinely shocking (and very representative of how Ninjas were perceived) if one of the stage hands were to pop up out of nowhere and stab someone. From this, we got the representation of Ninjas as wearing all-black outfits.