What makes blood the colours that it is?

In Liesmith, Lain has dark purple blood that turns dark green when exposed to air. It wouldn’t be due to haemorythrin or chlorocruorin, however, because Lain is flighted and thus would need the sort of super-efficient oxygenation those molecules aren’t capable of.

On the other hand, Lain’s blood also corrodes concrete and iron, so maybe it’s best not to think too hard about the science of gods, and instead just chalk it all up to, “It’s magic!”