Color helps to add identification when someone is talking outloud (compared to mere thougths). Also helps quickly note other people there is speech in your post and thus focus to read it (for some people anyways not all of them). Also it adds bit own flavor for your char speech posts for claiming color for it. Also if person (or GM) posts multiple chars speaking, it helps differ speakers with color (example main evil guy uses red color while minion captain and minions use their own colors).