Hemospectrum

Each troll has a different color of blood. Aside from Karkat, the color of their blood matches the color of the symbol on their shirt (and the color of their text on Trollian). Vriska has referred to Karkat's text as HS 4196 ("lame gray anonymity,") proving that all (or most) of the other trolls wear their blood colors on their shirts and on their text. The blood of a Lusus appears to be the same color as its associated troll, implying that it is a large factor in the selection of a lusus.

The blood colors are distributed at 30° angles on the HSV color system, varying by a few degrees here and there. The only color missing is pure green (120°), with Nepeta's blood (82°) lying eight degrees yellower than chartreuse (90°) and Kanaya's blood being spring green (150°). This is likely because the apparent difference between the three colors is smaller than between any other adjacent pairs. The colors are all fully saturated, but their values have a wide distribution.

There is a caste system on Alternia based on the color of an individual's blood. The hierarchy follows a rainbow, with red being the lowest and purple being the highest. Blue blood is shown to be a high caste color in HS 3961 (a conversation) between Karkat and Terezi. Equius describes Nepeta's green blood as "ok but not great". Sollux is quite low on the hierarchy and has a hard time thinking of a color of blood lower than his own yellow.

The trolls with the highest blood color of violet (Feferi and Eridan) are sea dwellers who live underwater, sport fin-like protrusions on the sides of their faces, and rule over the rest of the troll species. Gamzee, with indigo blood, is still a land dweller, but lives on the coast and has a sea-dwelling Lusus. Feferi is noted to have the highest blood color in existence, the same as that of the trolls' Empress, Her Imperious Condescension, and making Feferi the obvious heir apparent. It should also be noted, that Aradia is on the exact opposite end of the hemospectrum, making her blood the lowest in existence. Aradia refers to Tavros and herself (the two being the lowest on the hemospectrum out of the 12 Sgrub players) as "rust bloods".

Higher blood castes seem to have more lavish hives: both Equius's and Vriska's hives resemble castles as opposed to Aradia's humble hive. Both Eridan and Feferi wear jewelry, as well.

Kanaya's jade blood is quite rare and even warrants the protection of a Virgin Mother Grub.

When asked by Terezi what color his blood is, Karkat gets defensive; although the other troll's colors represent their blood colors, Karkat masks his in anonymity by using a gray tone. When Karkat arrives in the Land of Pulse and Haze, he appears to be surrounded by a river of red, which he believes to be comprised of his own red blood, a mutation that places him outside the "hemospectrum". It has yet to be revealed what significance this has, but it could have something to do with his Ancestor.

Initially some of the trolls believed human blood works on a similar system, although only Equius and Eridan seem to care much about the hemospectrum. Terezi is also very curious about human blood, specifically about how it tastes and smells.

It is possible that the blood-color caste system may correspond with medieval class systems on Earth. Based on this supposition, purple blood would be high royalty, blue would be nobility of sorts, green would probably be lesser nobility, and red would be peasantry. The intermediary colors are harder to discern but may correspond to other roles in feudal society or may fall under the previously listed four groups.

A Libra sign was prominently featured during the trial of Marquise Spinneret Mindfang by Neophyte Redglare, which may imply that the troll's signs have something to do with their (dream) professions. However, the Libra sign was teal, Redglare's blood colour, so may have been a vanity item of her own. Mindfang later associates the Cancer symbol with a rebellion.

Blood colour seems to have several biological consequences in addition to the social consequences of the cast system. Troll tears are apparently the same color as their blood, though more diluted in color. Trolls lower on the hemospectrum are more likely to have psychic powers, but are more psychically susceptible as a result.

Equius remarks that Gamzee has the highest blood possible for a landdweller and is as such more prone to violence and murder than the rest of them. He also states that he dreads to think what he would be without the pacifying influence of his morail, Nepeta. This implies that trolls gradually become more psychopathic/violent the higher they are on the hemospectrum. This would mostly be in line with the twelve trolls followed in Hivebent, as the trolls below Terezi on the spectrum are fairly "harmless", while from there onwards the trolls display some unnerving behaviour peaking in Eridan's outright genocidal tendencies. The only true exception (since Equius dangerous side is largely pacified by Nepeta) seems to be Feferi, who despite being at the very top of the hemospectrum displays no negative effects. Relating to this thought it might be worth noting that Aradia starts displaying more violent behaviour after entering her blue blood fueled Soulbot.

Mindfang's journal also implies that higher-blooded trolls enjoy longer lifespans, she expecting to live for centuries and predicting the Summoner (Tavros's Ancestor) destined to kill her may very well not hatch for many sweeps given the shorter lifespan granted by his lower position on the hemospectrum. It is revealed HS 5968 (here) that the lowest-blooded trolls usually live for a dozen, or a couple dozen, sweeps (roughly 25 to 50 years in human terms).