There's a few different ways in which Moodle marks up short status messages e.g. I've found the following,

span.ok,
span.statusok,
span.error,
span.warn,
span.statuswarning,
font[color="green"]

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:

http://twitter.github.com/bootstrap/components.html#labels

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.