did you use the color like this? if not, then it's like
[color=#fffff]word[/color]
edit: but the #ffffff part is a hex code (ex: #00ff89, #ff0087), and to write it literally like nothing, then it's the color of the forum page that the person is on. if you're talking about the black bar that hides a word, then i have no idea and you need to ask someone else.