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. 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.

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).

Candy Red Blood
Candy Red Blood is an extremely rare mutation on Alternia. Karkat Vantas and his Ancestor The Signless/Sufferer are the only two trolls to ever have the blood color and fear (or faced) ostracization based on it. 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.

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. 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.

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 physically weaker than most trolls as well, but again said to be more likely to develop psychic powers.

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).

Yellow Blood
Yellow Blood is the third-lowest blood color. They are weak but are more likely to have psychic powers to make up for it.

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
Nothing is currently known about Lime Blooded trolls, as there was no lime blooded troll amongst the group of 12 Hivebent Sgrub players. The only inferences to the existence is that of a blood stain on the walls of Grand Highblood and a statement by Andrew Hussie on his Tumblr account.

The color on the Highblood's wall, however, was only added in retroactively, as it was a much darker shade of green in the original Grand Highblood page than it was later changed to. This new color occupies the space between Sollux and Nepeta, but astrologically this space is occupied by Karkat. The original color of the blood was an intermediate between Nepeta and Kanaya. The sudden change and mentions by Hussie suggest an upcoming significant character who is a Lime Blooded troll, with the yet-to-be revealed identity of uranianUmbra as a possible candidate for this spot. It is also possible that Karkat and the Sufferer would have been lime bloods were it not for their mutation, given how lime is between yellow and green on the hemospectrum in a similar manner to how Cancer is between Gemini and Leo.

Green Blood
Green Blooded trolls are 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, but both were able to prey upon Alternia's otherwise dangerous fauna. This may indicate that somewhere in the green part of the Hemospectrum trolls start to be more likely to have physical strength over psychic powers.

Jade Green Blood
Jade Green Blood is rare among trolls, and trolls in this part of the Hemospectrum are of high prestige 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 physically stronger than most trolls, and 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 have Cerulean Blood. However, both are also very powerful psychics who are able to manipulate others minds to bend to their whim. Both also have vision eightfold, allowing them to peer within the Magic Cue Ball to discover its secrets. It is not known if some of these traits are simply unique to the Paradox Clones or are common amongst the blood color.

Indigo Blood
Indigo Blood is the second-highest tier of blood amongst the land-dwelling trolls. They are implied to be stronger than most trolls and are considered part of the nobility, and as such they have a traditional rivalry/mutual-kismesissitude with the sea-dwelling trolls. As with cerulean blood this blood type is often simply called "blue", which is however unspecific.

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. He is also capable of regenerating his teeth and horns. While Equius's strength would enable him to be a Ruffiannihilator, he prefers to be an Archeradicator, the elite archers of the imperial forces, a role shared by E%ecutor Darkleer (although more indicative of their Sagittarius-themes). 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). 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. Violet Blooded trolls tend to be more violent than the others, 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 (resembling Tyrian/Royal Purple) Blood 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 she speaks, 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 in existence 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 is also implied to have Fuchsia blood (given her shirt design and such), though it may be because she is probably the pre-Scratch Condesce.

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.