If you find ways of doing it roundabout, you can have characters in RPs be a bit racist (or the more fun version, speciesist). Its common for various races/species to show scorn or worse for others. But instead of just white ganging up on black, you could go with black and white ganging up against the green. We'd still do as Jorick recommended whenever you're not posting IC. Mind you, we personally like avoiding profanities even OoC, but thats a personal preference.
If you absolutely feel the need to insult someone and can't restrain yourself, do at least try to be creative about it. Mind you, not everyone are tough-skinned when it comes to insults. Thus it is generally best to avoid things that can be considered insults against the person behind an account.