I'm going to compare three separate narratives I've recently watched that I enjoyed almost equally, but for different reasons. A couple are from Madhouse, which I think is interesting because they're quite a contrast, separated by five years. Even animation-wise, they are distinct and yet, still, use this technique involving colour - for different reasons, though, it seems. Shiki is by Daume, but since I haven't yet seen anything else by them I can't compare anything more, except for these three as distinct narratives from the same genre and topic.
But, this is mainly about Hitsuji no uta, although I think comparisons would be worth it since the symbolisms and ramifications of all three have different consequences.
The 'lamb' in the title, I assume, refers to the docile nature of the protagonists, this time. Whereas in both Kurozuka and Shiki (and most other media I presume, stereotypically), vampires are usually striving to survive, not unlike any other living creature, and although they are 'undead', as opposed to 'zombies' with which I assume they share this fictional 'status', vampires are usually conscious and thoughtful, as they are in all of these three titles.
What separates Hitsuji no uta from the others, though, is that it is almost an 'experiment' about going against nature. Unlike the other titles and most others, the couple of protagonists, or one could say, their entire family, struggle to not give in to their urges. Whereas in Kurozuka history is combined with fiction to create a hyper-aggressive narrative against all odds and times, where they strive to survive no matter what, in Shiki there are a few timid vampires, but the majority do not fight their nature. In the former, a sci-fi sort of atmosphere dominates, where Madhouse skilfully uses grays and black-and-whites to convey an alternate reality where the source of vampirism seems to take over, in Hitsuji no uta the changing of colours indicate something slightly different and subtler. On one hand, they do use it to portray the past as much media already does, but it differs in that it uses a similar, but slightly differing scheme to paint a picture of an internal struggle, or even to simply convey hopelessness (with buildings, like with Brutalist architecture, I think it's meant to convey a sense of permanence perhaps, as opposed to the temporal counterparts that creatures alive represent).
All three use colours for their own reasons, though, as I suppose most animation does, except that in this case, as all three can be categorized as 'horror', the colour is used specifically to either intensify such 'pictures' that the animators presumably wanted to convey, or lower the tone, that carry with them feelings of one sort or another. In Kurozuka e.g. red must be the predominant colour, although that might seem obvious in a vampire's narrative, since it's the colour of blood, but that series wants its watchers to know that it's designed to be aggressive, which doesn't necessarily preclude a certain artistic possibility, with its inclusion of traditional Japanese motifs and Kabuki, which further references prior media about the same sort of history, and yet from an entirely different perspective (Kurosawa).
In Hitsuji no uta, though, colours are subdued, very much like the characters themselves and the themes contained within the narrative, not unlike Boogiepop Phantom, which isn't about vampires and yet contains almost identical themes (with the exception of a narrative derived from the 'dementia' genre). This sort of technique in anime in general has a pattern, wherein the more fantastical a narrative, the more colourful. It is, as such, similar with Shiki in the opposite direction of Hitsuji no uta. Whereas with the latter it is mostly two siblings that feature prominently and the narrative revolves about (and due to dark themes, the animation is also dark), in Shiki there aren't really any central characters that feature in every single episode. It is, essentially, about an entire village of both humans and vampires, and as such an ensemble drama that involves a variety of personalities that not only reflect in their wildly differing hair colour, but hair styles, and, ultimately, their intrinsic motives. Both series are about the same topic, and both could be said to be sociological and the implications thereof, but they are approached from distinct angles. Hitsuji no uta is more inter-personal, more confined to a few individuals. Society has an effect on the narrative, similar to the complications that arise in Shiki, but whereas the latter is more about society's moral ambiguities, in Hitsuji no uta society is in the background, and yet still affects the protagonists.
It could be said that, generally, with this genre and topic, it serves to reflect an already existent undercurrent in human societies. Whereas vampires are fictional, and yet historically, as with witches and werewolves, moral panics existed in which people somehow convinced themselves and others that they truly are real, whether intentionally used as an excuse to justify the crushing of opponents, or simply a genuine delusion borne out of times when even biology was little known, ended up destroying non-supernatural lives. Ultimately, it is a psychological manifestation that combines imagination with possibility, and despite these kind of series being entirely fictitious themselves, they could almost be a documentary on the many facets of a psyche.
