Have you tried using a different browser? Some browsers don't show the advanced editor.
If you're browser usually shows it but suddenly stopped, then you might have to change the settings here (Miscellaneous Options - Message Editor Interface) to Enhanced Interface - Full WYSIWYG Editing. If that doesn't work on a browser that normally shows it then it's probably sending a wrong useragent for some reason.
You can type [COLOR=******] Text [/COLOR] replacing the *'s with a hex color code from here if you're browser doesn't show it.