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Trolls' distinctive features are their grey skin and horns. Their horns have red-orange bottoms, with their color progressing gradually to yellow at the tips. These horns can grow in a number of different shapes, sizes, and even numbers, and are usually fairly distinct between individuals.
Trolls have dark yellow reflective sclera with black irises that fill in with their blood color as they age. They also have pointed teeth, yellow nails, black lips, and black hair.
An individual troll may possess one of several different colors of blood spread across the color spectrum. This is known as the Hemospectrum, and it is the basis of a stratified societal caste system that assigns social status based on blood; low ranking trolls are regarded as "Lowbloods", whereas high ranking trolls are referred to as "Highbloods". Different blood castes also have differing biological aspects, such as the high psychic ability among Lowbloods, and the gills possessed by the high-blooded ruling Seadwellers. Natural lifespan also increases in line with one's rung on the Hemospectrum.
Young trolls have always been shown have first and last names with six letters each, while older trolls have been shown to acquire adult names or titles, typically with 8 letters, although there are exceptions.
Each troll has a sign, typically depicted on their clothing in some form, which is assigned from an alphabet of signs for each Hemospectrum caste.
In Homestuck, the signs of the twelve main trolls became the signs of the human universe's zodiac. They were introduced at first as a group of twelve internet trolls, who often bothered the kids on Pesterchum before being revealed as literal trolls. These particular trolls are approximately the same age as the kids and are the protagonists of Hivebent, which is the first half of Act 5 of the Homestuck.
Troll culture has a distinct romance system - see Troll Romance.
Homestuck cosplay became extremely notorious in conventions because of the shenanigans for fans, particularly the wardrobe malfunctions of Troll cosplayers.
The most notorious of these issues was that of "going grey," when a cosplayer paints all of their visible skin grey to match the canon skintone of the trolls. This process became known from being extremely messy, often because cosplayers were inexperienced. The paint often rubbed off and damaged costumes, the property of others, or the convention space.
There are legend-like horror stories of people who went grey improperly. Perhaps the most notorious one is about the girl who tried to dye herself grey by soaking in a bathtub full of alcohol and Sharpie dye. This took place at Anime Expo 2011 and the cosplayer was reportedly kicked out of the hotel and paid $700 in damage fees. The full story was posted by a Tumblr user in 2012 and shared widely .
A number of tutorials and fandom advice posts circulated widely to try and assist other cosplayers in not being a nuisance. Commonly found with "going grey" was the directive to "seal your paint"; since theatre-grade creme paint was frequently used, tutorials directed cosplayers to apply the paint in layers, seal with translucent setting powder or baby powder (though the latter would result in a lighter finish), and then finish with either hairspray or setting spray. Following these sorts of tutorials usually resulted in an even application of the paint which would last throughout the day and resist smearing off. Another method included using water-based makeup, though this was less popular because of frequently streaky finishes.
Where cosplayers would have arms exposed, PAX body paint was sometimes used to ensure the body paint would last: a mixture of Pros-Aide and acrylic paint which would dry and could be set with powder and spray . However, this method sometimes required shaving where it would be applied to avoid the paint sticking to hair, and regardless of shaving it was the most difficult method of body painting to remove. More commonly, cosplayers would create armsocks - arm-length gloves created from solid colour tights 
Some of these tutorials were later used for other fandoms where characters had unique skin tones, such as the gems from Steven Universe.