Oookay so well today I remembered that certain sea creatures have blue blood instead of red, and after doing a little research on the subject, I realized that the entire troll hemospectrum could possibly be feasibly explained with different amounts of the different proteins hemoglobin, hemocyanin and hemerythrin! Sorry if it’s a little too sciency for one’s tastes but do let me know if I’ve gotten anything wrong!
(If it’s something like ‘individual organisms can’t produce more than one oxygen-binding protein at once’ well my friend please do remember that the subject matter is a species from a different universe that breeds in a similar manner to ants, bees and termites, and we can probably find a way to work around it anyhows)
Zcience!
Xenobiological headcannon accepted
I know this may go unnoticed, but some peers and I, upon reading this explanation, have proposed something of an extension. The blood of trolls is actually much darker than the blood of humans, so it could be possible that trolls have a some sort of darker, melanin-esque pigment present in their blood as well.
If this is to be assumed, then mutants would not only have a malfunction in which pigments are produced –which in a quick side not can be from any one caste to another as all trolls would be genetically capable of producing any of the blood pigments–, but they would not produce sufficient counts of melanin, creating a case similar to albinism in humans and other known animals. This would give Karkat a similarity with Dave, as the only natural means of having red eyes is being albinistic, at least for humans anyways, meaning that in a sense they are both albinistic, red-associated knights.
On the opposite end of albinism is melanism, or the over-production of melanin. This would provide some sort of explanation for the /highly/ mutated troll, if he is one at all, known as His Honorable Tyranny, given that he bears, at least by the account that we have seen, black blood.
I hope this was not off in any way, as we tried staying as accurate as we could, and if we made a mistake somewhere, feel free to point it out. Sorry that we could not prepare a fancy chart like the original post.