Or: Why Karkat's life sucks, rainbow edition.
Trolls have a myriad of blood color. As always in society, anything that can be used to set you apart is, and since trolls don't have PROPER CAPITALISM or DIFFERENT RACES, troll classify themselves using their blood color. The lower on the hemospectrum you are, the lower you are considered "class" wise. The higher you are, the better your perks, etc. This is basically a fancy way of saying the trolls have a caste system based on blood color.
The troll hemospectrum varies between nautical purple, which is the very highest, and a very dark red, which is the lowest.
The troll hemospectrum is indeed a goshdarned rainbow.
Trolls from the lower end of the spectrum are more likely to have psychic powers, but they are also more psychically subjective. Trolls on the higher end of the spectrum, meanwhile, get more psychopathic/violent tendencies the higher they go. Trolls from the higher end of the spectrum also get a much longer lifespan than trolls on the lower end of the spectrum. This is a case of living centuries vs a normal lifespan, presumably.
A troll's LUSUS, or guardian, also shares a troll's blood colour, which implies blood color is related to the choosing process. This means nothing to anyone until I do my in-depth guide on troll culture, so whatever. IGNORE ME.
A troll's blood color is also the color of their tears, the color they will blush, they color their body were when they were grubs, etc. Their previously grey irises will even, when they attain troll puberty adulthood, fill in with the color of their blood.
So there is really no point in being shy about blood color. Thus, trolls usually wear their astrological sign in, and type in, their blood color. They literally wear their blood color on their sleeves.
LET US NOW EXEMPLIFY EVERYTHING USING COLORS FROM THE HOMESTUCK TROLLS THEMSELVES.
This color is really low! It is considered part of the "rust bloods". You pretty much cannot fall lower than this. . .
This is still pretty low, but a bit higher! Again, you are considered a rust blood. Fans assume that rust bloods include dark red to orange!
This is still low! Sollux himself has a hard time thinking of someone lower than yellow, so rust bloods are kind of the troll equivalent to HP mudbloods, at this point.
All shades of green are considered pretty middle ground. Nepeta's blood (this color), for example, is qualified as "okay but not great".
This shade of green is special! Jade Green is EXTREMELY RARE in trolls. It means you get a very special lusus and blablabla.
Teal is considered too green for the bluebloods taste. While it is the highest green you can get, it is not considered blue!
Now you're getting somewhere! With this shade, you pretty much get to build yourself a castle. I am not even kidding.
This is so high you start to think you are important.
THIS IS THE HIGHEST POSSIBLE COLOR A LAND DWELLING TROLL CAN GET.
You are now a seadwelling troll. You have gills. You are pretty much royalty. Life is pretty sweet!
THE HIGHEST POSSIBLE COLOR FOR A TROLL EVER. EVER.
MOVING ON FROM THE GENERIC BASIS, Karkat is thus an anomaly by trying to keep his blood anonymous. This is because Karkat's blood color is NOT ON THE HEMOSPECTRUM AT ALL.
Karkat has bright red blood (like humans), and is thus a DIRTY MUTANT. This means that, if anyone were to find his blood color, he would be culled. This is a fancy word to say he would be killed faster than you can say "mutant". And this is why Karkat is very careful not to bleed/cry/etc in front of people and remain a dirty grey anonymity. :)
PRETEND THIS MADE SENSE.