From what I've seen in roleplays and in the stories that were submitted for the writing contests: dialogue is done in plain text, but in some roleplays people decide to give the lines of dialogue a colour to make it stand out in the rest of the text. Not everyone does that though, I personally always use plain text and use italics for thoughts, emphasizing words, or when someone speaks in a different language than the one used by the main character(s) in the RP.