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.

Social Aspects
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. There seem to be several terms covering chunks of the hemospectrum. Aradia refers to Tavros and herself (the two being the lowest on the hemospectrum out of the 12 Sgrub players) as "rust bloods", while Terezi, Vriska and Equius have all been refered to as "blue 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.

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.



Biological Aspects
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. Another obvious aspect is the different anatomy of land- and sea-dwelling trolls.The latter sporting fin-like protrusions and organs allowing them to live underwater.

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).

Information about each caste by color
Note: Karkat's mutant red blood is excluded because it is not actually part of the Hemospectrum.

Dark Red Blood
The lowest blood caste in existance, they are notable for having the shortest lifespans. They are the weakest trolls physically, and would be the first to die in the event of the Vast Glub. To make up for this physical weakness, they are more likely to have psychic powers.

Brown Blood
The second-lowest class of blood, not much has been explained about them, since the only two who appear are notable special cases gifted with "vision twofold". They are physically weaker than most trolls as well, but again said to be more likley to develop psychic powers.

Yellow Blood
The third-lowest blood color, they are weak but are more likely to have psychic powers to make up for it.

Green Blood
Considered the "middle class", and is referred to as "ok but not great" by Equius.

Jade Green Blood
Kanaya's jade blood is quite rare and even warrants the protection of a Virgin Mother Grub. Rare among trolls, trolls with jade green blood are the only ones capable of surviving under the Alternian sun, which is dangerous for others. Because of this, they are raised by Virgin Mother Grubs, who abandon their posts as Mother Grubs to do so. In the past, those with jade green blood were forbidden to leave the caverns they were born in and served the Mother Grub, who in turn presumably acted as their Lusus.

Teal Blood
Considered a high class, it is the closest rung to nobility.

Cerulean blood
The first rung of nobility. It is suggested that they are physically stronger than most trolls due to their blood.

Blue Blood
The second-highest tier of blood in the landwelling trolls. They are implied to be stronger than most trolls and are considered nobility, and have a traditional rivalry/mutual-kismesissitude with the sea-dwelling trolls.

InDiGo BlOoD
The highest blood possible among land-dwelling trolls, thus often refered to as high bloods". Gamzee lives near the coast and has a sea dwelling Lusus, but it is unclear if this is a constant in this caste. They are incredibly violent and their actions can be unpredictable, making them incredibly dangerous. For example, The Grand Highblood was shown with walls covered in every blood color from hemospectrum but his own indigo blood and the mutant red blood of Karkat and the Sufferer. For this violence, in the past the Condesce had them serve as "Subjugglators" to exact her vengeance. It is unknown Gamzee Makara is a rare exception to this, thanks to the mind-numbing effects of mind jelly.

Purple Blood
The second-highest type of blood, it's the blood of sea-dwelling trolls. These trolls tend to be more violent than others, and it's traditional for them to hate land-dwelling trolls.

Tyrian Purple (Royal)
The highest and rarest blood of all, those who possess it are sea-dwellers and considered royality and the ruler of Trollkind. It is implied in HS 4341 (Feferi's introduction page) that there are only two trolls who may possess it at any given time: The current regent, who is off conquering worlds, and her heir apparent, a young troll who lives on Alternia. The two share a lusus, Gl'bgolyb, which is the only lusus with the same blood color. The Condesce would try to kill the successor. stated, HS 5963 ("Heiresses upon reaching maturity were expected to challenge the Condesce for the throne. It was not merely expected of them by their people, but demanded by their shared lusus.") (It is notable that it refers specifically to "heiresses", making it possible that all who possess this blood is female.) Feferi was a rare exception to this, as The Condesce had steered her flagship out of the range of their lusus's psychic connection, making Gl'bgolyb switch her allegiance to Feferi.

Trivia
The blood colors of the main 12 trolls 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 hues have a wide distribution.