Hair is a fairly simple matter. Some consider colored hair mary-sue, but I personally think that a streak in the troll's blood color or white hair isn't too far overboard.
Clothes are also pretty loose as far as 'rules' go, but For the most part; Trolls tend to wear mostly black with their symbol and/or accessories/embellishments in their blood color. Red, white, grey and bluejeans have been shown, as well as gold rings or the color of someone you are in a relationship (Nepeta's blue hat and tail)
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Fantroll Guidelines
RandomDisclaimer: 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 :)