Trolltags mostly follow the GCAT rule, meaning the initials are G, C, A, or T. The 3 exceptions are Caliborn and Calliope, who's chumhandles begin with UU and John, who is EB, ectoBiologist. The text color is usually the blood color (trolls) or the eye color (humans) there are, once again exceptions: Karkat's grey, and Caliborn and Calliope's shades, though they were changed in the timelines where one or the other dies. This is clearly not an extremely important thing, just common antics. Never set in stone.
Final note: biological troll/human/carpacian/whateverthefuck else hybrids are really marysue!!! even if it's through ectobiology, it will still (probably) be considered over-the-top.
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Fantroll Guidelines
De TodoDisclaimer: I am not positive about all of these, so please don't hate if I get something wrong. This is just some basic guidelines from what I know. If you think something is incorrect, feel free to let me know politely :)