I don't know of any way to do this with a Content Control.

Consider using a MacroButton Field instead. You can format the prompt language for one of those (for instance in red bold comic sans) and use whatever prompt you want. The prompts can't wrap to a different line, though. This is the equivalent of a Content Control that disappears when you type in it.

Macrobutton fields have the advantage that they will work in any version of Word, at least back as far as Word 97.