There's a few different ways in which Moodle marks up short status messages e.g. I've found the following,
and I'm guessing there's more about e.g. font red for warnings.
I'd suggest standardizing on the format used by Twitter's Bootstrap and then re-using their CSS:
Once that's done there's other occasions were re-using this same markup might be useful e.g. if you override things in the config.php we could use span.label.label-warning etc.