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 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. Andrew Hussie has confirmed that there are only 12 possible blood colors, excluding mutations.

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) 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' 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 text 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 his Ancestor suffered from the same mutation.

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 color seems to have several biological consequences in addition to the social consequences of the caste 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 sport fin-like protrusions and organs that allow 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. Equius also states that he dreads to think what he would be without the pacifying influence of his moirail, 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's 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. Either that, or she tries very hard to suppress it, since she was screaming while attacking an imp HS 4478 (here). Relating to this thought it might be worth noting that Aradia starts displaying more violent behaviour after entering her blue blood fueled Soulbot.

In a dreambubble conversation Terezi had with Aranea, who was masquerading as Vriska at the time, it was implied that high blooded trolls are tougher than low blooded trolls and likelier to survive grievous injuries. In the same conversation, Aranea states that the trolls' violent behavior is a result of 's conditioning of their race, implying that the high-blooded trolls may not be as unstable as was previously thought.

Mindfang's journal also implies that higher-blooded trolls enjoy longer lifespans, with Mindfang 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).

Candy Red Blood
Candy Red Blood is an extremely rare mutation on Alternia. Karkat Vantas, his Ancestor The Signless/Sufferer, and undyingUmbrage are the only three trolls known to have this blood color. Karkat and his Ancestor fear/faced ostracization based on their mutation. uu's present situation seems to render his mutation free from negative consequences for him. Due to its rarity, there was no Lusus in the Sufferer's time that would choose him and the Dolorosa raised him herself; Karkat was only afforded a Lusus due to the actions of the followers of the Sufferer, who prepared for the birth of another Candy Red Blooded troll due to the prophecies told by the Sufferer. It has also been called "bright cherry blood" by UU and Terezi associates its "smell" with cherries.

The abilities afforded to a Candy Red Blooded troll are not shown. Unlike other lowbloods, Karkat is never seen exhibiting psychic powers. Karkat is also shown to have a great knowledge of the way troll Relationships work, and both he and the Sufferer have managed to form bonds that exist outside of the four known quadrants. Both have also managed to unite trolls of different blood colors against a common threat.

Dark Red Blood
Dark Red Blood is the lowest blood caste in existence. They are notable for having the shortest lifespans - said they generally live only a dozen sweeps or two, or roughly 30-60 human years. They are the most likely to have psychic powers. This is a double-edged sword, however, as they are also the most psychically susceptible, and would be the first to die in the event of the Vast Glub.

Aradia Megido's psychic powers, prior to her Flarp-related death, allowed her to commune with and command the ghosts of trolls to do her bidding. After her death, she continues to exist as a ghost and her psychic abilities give way to stronger psychic powers afforded to her by her status as a ghost. The Handmaid is also known to have such psychic abilities, though she was only seen exhibiting them as a child.

Brown Blood
Brown Blood is the second-lowest class of blood. They are more psychically susceptible than most trolls as well, but again said to be more likely to develop psychic powers. Trolls of this caste have also been called fudgebloods, and Terezi associates the blood's smell with chocolate.

Both Tavros Nitram and his Ancestor The Summoner have the psychic ability to commune with and command the various beasts of Alternia (and in Tavros's case the Underlings of the Incipisphere, possibly due to their Lusus prototypings).

Troll Kevin James has brown bloodHS 4304.

Yellow Blood
Yellow Blood is the third-lowest blood color. They are also very likely to have psychic powers.

Both Sollux Captor and his Ancestor The Ψiioniic/Helmsman are powerful telekinetics, capable of using their mental powers to move massive objects at high speed; the Helmsman himself was used to power the Condesce's own battleship, making it the fastest out of all ships in her fleet. They are also gifted with "vision twofold", but this may merely be a trait unique to the two Paradox Clones. In addition, both Sollux and the Ψiioniic/Helmsman die due to the Vast Glub released by Gl'bgolyb while they are directly serving their Tyrian Purple Blooded companions.

Lime Blood
Lime Blood was a very common blood caste but was hunted to extinction by the others due to its members' possession of powerful abilities that threatened the authority.

uranianUmbra is the only known lime blooded troll character, but because of the game she is playing with undyingUmbrage she does not elaborate any further to Jane, other than saying she may have the powers she alludes that lime bloods tend to have. Being a lime blood may also relate to how she uses British and Commonwealth spelling of "-our" over "-or" ("honour" instead of "honor", "favour" instead of "favor"), as an old derogatory slang for the British is "limey".

The lime blood color was originally alluded to when the Grand Highblood was introduced, as there was a shade of green previously unaccounted for. The existence of the color was later confirmed by Hussie, and HS 5414 (the page) was retroactively changed to feature a lighter shade of green. This lighter shade fits in the color wheel between Sollux's yellow and Nepeta's green, where Karkat sits when the group is arranged by the procession of their zodiac symbols, suggesting that he (and by association the Sufferer/Signless and undyingUmbrage) would be a lime blood if not for the cherry red blood mutation. There is another color which was shown on the page where Calmasis was chained up in the Complacency of the Learned movie. This web color is called Harlequin. Due to the fact that undyingUmbrage called uranianUmbra's blood "harlequin slime", and because the color displayed on her death page is darker than the actual color of her blood due to Skaia filling up with dark clouds, this is likely her actual blood color. Supporting this is the color of her chat symbol.

Green Blood
Green Blood is considered the "middle class", and the color is referred to as "ok but not great" by Equius. "Green" may not be the specific term, as there are other shades of green on the hemospectrum. "Olive" has been used to describe a potion of the same color, which can be assumed to be (based on) Nepeta's blood.

Nepeta Leijon and her Ancestor The Disciple are not known to have any sort of psychic power. This may indicate that somewhere in the green part of the Hemospectrum trolls start to be more likely to be born without psychic powers.

Jade Green Blood
Jade Green Blood is rare among trolls, and trolls in this part of the Hemospectrum are given a unique role in troll society. In the past, Jade Green Blooded trolls were tasked to tend to the Mother Grubs in the birthing caverns, while in modern Alternia their rarity bestows upon them the rarest and most prestigious of Lusii.

Kanaya Maryam and her Ancestor The Dolorosa are the only Jade Green Blooded trolls described in Homestuck. Both seem to be able to withstand the intense light given off by Alternia's sun. Kanaya, who is afforded the privilege of being brought up by a Virgin Mother Grub ("an event so rare as to elude documented precedent"), also eventually turns into a rainbow drinker (the Alternian equivalent of a vampire who can withstand sunlight and radiates light) upon her death at the hands of Eridan. It is not known if the Dolorosa shared this ability, as Mindfang's narrative does not convey any details about her after her assassination.

Teal Blood
Teal Blood is considered a high class in Alternian society. As a chromatic median between the green and blue colors, Teal Blooded trolls are not quite considered nobility, but still outrank the middle class Green Bloods. Trolls of this blood class typically take civil servant positions.

Terezi Pyrope and her Ancestor Neophyte Redglare both fulfill their roles as Teal Blooded trolls. Neophyte Redglare was a legislacerator, the Alternian equivalent of a lawyer who also doubles as a bounty hunter, and Terezi hoped to one day also have such a role in the Alternian legal system. Both also had dragons as their Lusus, which Mindfang stated was quite unusual for a troll of their blood color to have, and used their verbal prowess to affect others, rather than any sort of psychic ability.

Cerulean blood
Cerulean Blood is the first rung of nobility. It is suggested that Cerulean Blooded trolls are unlikely to develop psychic powers. Usually this blood colour is simply refered to as "blue", which is however not very specific. Vriska herself called her blood "cerulean" once. However a potion presumably based on her blood is refered to as "cobalt".

Vriska Serket and her Ancestor Marquise Spinneret Mindfang, as well as her pre-scratch self Aranea have Cerulean Blood. All three of them are very powerful psychics who are able to manipulate the minds of others to bend to their whim. All of them also have vision eightfold, allowing them to peer within the Magic Cue Ball to discover its secrets. Aranea confirmed that psychic abilities related to mind reading and manipulation are an occasional phenomenon amongst cerulean blooded trolls. It is not known if this goes hand in hand with vision eightfold, or if this trait is simply unique to the Paradox Clones.

Indigo Blood
Indigo Blood is the second-highest tier of blood amongst the land-dwelling trolls. They are considered part of the nobility, and as such they have a traditional rivalry/mutual-kismesissitude with the sea-dwelling trolls. They are prone to violence as well because of their status in the caste. As with cerulean blood this blood type is often simply called "blue", which is rather vague.

Equius Zahhak and his Ancestor E%ecutor Darkleer are both Indigo Blooded (or as they would say "b100 b100ded") trolls. It is stated that Equius's enormous strength is unique to himself, rather than being characteristic of the Indigo Bloods. It is unclear if the same goes for his ability to regenerate his teeth. While Equius's strength would enable him to be an effective Ruffiannihilator, he would prefer to be an Archeradicator, the elite archers of the imperial forces, a role shared by E%ecutor Darkleer (although more indicative of their Sagittarius theme). Both are also skilled in robotics. Darkleer is also inherently shrouded in a Void, although this may be due to Equius's role as the Heir of Void. Aradia also became Indigo Blooded when her ghost inhabited the soulbot Equius constructed for her, and she became much more violent, a trait absent from the lowblooded castes, having been given emotions once more.

Purple Blood
Purple Blood is the highest ranked blood possible for land-dwelling trolls, and are referred to as the "high bloods". Because of their blood color, Purple Blooded trolls are more prone to extreme violence and unpredictability, and it was for this reason that most were tasked as the violent Subjugglators under the Imperial Condescension. Andrew Hussie stated that all the Purple Blooded trolls are adherents to an Alternian Juggalo-like cult that believes that "a band of rowdy and capricious minstrels will rise one day on a mythical paradise planet." Hussie also suggests that Purple Blooded trolls are granted psychic powers (the "chucklevoodoos"), which permits them to keep the lower blood colors in check.

Gamzee Makara and his Ancestor The Grand Highblood are the only Purple Blooded trolls that are of note in Homestuck. The Grand Highblood served as a Subjugglator, while it was Gamzee's ultimate goal in life to also serve this purpose. Both characters are also highly violent and unpredictable, but it is only after Gamzee runs out of the narcotic Sopor Slime that his true Purple-blooded nature arises. It is notable that Gamzee used an immense amount of psychic power, after becoming sober, to make the Kids' session "terminal" (by creating a harlequin doll missing an arm and an eye in John's Dream Self's room, causing a chain reaction of events). As it is a median color between the blues and the sea-dwelling purples, Gamzee is shown to live on a shoreline and has an aquatic Lusus. The only other Lusus that is shown is a whale with Purple Blood, suggesting that all Purple Blooded trolls have aquatic Lusii, but live on land.

Violet Blood
Violet Blood is the second-highest color of blood in the Hemospectrum and is the first of two that exclusively belongs to sea dwelling trolls, noted for their ability to breathe underwater, having fin-like protrusions from their neck, and having a collapsing and expanding bladder based aquatic vascular system (which is merely a fancy term for gills). Violet Blooded trolls tend to be more violent than the others seeing as they are high in the caste, and express an extreme hatred towards the land-dwelling nobility. Karkat calls this blood type "purple" once, this may however have been a goof.

Eridan Ampora and his Ancestor Orphaner Dualscar are the Violet Blooded trolls introduced in Homestuck. Both are violent towards the land dwellers, and sought a Kismessitude with their generation's Cerulean Blooded member and a Matespritship with their supreme royalty.

Fuchsia Blood
Fuchsia Blood (resembling Tyrian/Royal Purple) is the highest and rarest color of blood in the Hemospectrum. Like their plain Violet Blooded comrades, they are sea dwellers and are considered royalty. The blood color is so rare that those who possess the blood color are destined to be the sovereign of all trollkind.

Only two trolls at any time have this blood color, an older one serving as the Empress of the trolls and a younger heiress to the Alternian throne; this is due to the Condesce killing her descendants once they've come of age. Both are linked to the same Lusus, Gl'bgolyb, known as "The Rift's Carbuncle", "Emissary to the Horrorterrors" and "Speaker of the Vast Glub", who HS 5963 (compels) the younger troll to challenge the older for control of the throne.

Gl'bgolyb must be kept quiet by the Fuchsia Blooded Troll, as Gl'bgolyb's voice is capable of killing trolls in ascending strengths and numbers based upon how loud it is raised, with the lower castes susceptible to anything just above a whisper, to the ultimate Vast Glub which can kill all trolls in the universe save for the Fuchsia Bloods.

Feferi Peixes and her Ancestor The Imperial Condesce, are the only two Fuchsia Blooded trolls alive during the events of Homestuck, with the Condesce serving as Empress of Alternia off-planet leading the conquering forces, and Feferi staying on Alternia to tend to Gl'bgolyb. Because the Condesce is far away from Alternia, Gl'bgolyb's allegiance shifts to Feferi as the psychic connection is severed due to the distance.

The Condesce is also given a much longer lifespan, and she can also extend the lifespans of others. It is unclear if these abilities are related to her blood color or her pre-Scratch self having been a Hero of Life.

Due to the events of Sgrub, neither the Condesce or Feferi are capable of preventing Gl'bgolyb from releasing the Vast Glub, killing all trolls in the universe save for the Condesce, Feferi, the Handmaid (whose meteor lands in the future rather than the past), and the Sgrub players who manage to escape into the Incipisphere. Notably, Sollux and the Ψiioniic/Helmsman die due to the Vast Glub, but Sollux's multiple Dream Selves let him survive to play Sgrub.

Meenah being the the pre-Scratch Condesce also has Fuchsia blood.

It should also be noted that another lusus with HS 4361 (what appears to be) fuchsia blood is fed to Gl'bgolyb. Whether or not this has any significance to trolls with possible fuchsia blood could be debated.

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.
 * The word hemospectrum is a portmanteau of the terms hemo- (blood) and spectrum (band of colors).
 * The only other Alternian native who has been featured is His Honorable Tyranny. However, his blood is HS 5654 (pure black).
 * Nepeta's walls are covered in red blood, indicating that most of the Alternian fauna have red blood, and that the hemospectrum is unique to the Trolls and their Lusii.
 * It's possible that the variation of blood color itself is based on the blood pigments used by arthropods in nature (upon which trolls appear to be based). The predominantly land-dwelling insects use hemoglobin, which is a familiar red, while horseshoe crabs and many of the mostly sea-dwelling crustaceans use hemocyanin, which is dark blue when oxygenated.
 * The association between Alternian royalty & highbloods and the sea probably comes from the nature of the last three signs of the zodiac (upon which the trolls are based) - Pisces (the fishes) and Aquarius (the water bearer) are entirely aquatic concepts (just as Feferi and Eridan are entirely sea-dwelling), and Capricorn (the sea-goat) is a mixture of land and sea creatures (just as Gamzee lives by the sea, but not in it).