google don't expicitly state what they consider hidden text or how they detect it. If they did it would just lead spammers to find new workarounds.
The best thing to do is use some common sense. If the text is hard to see in a browser then don't do it unless you're prepared to explain to google why doing it this way helps people viewing your site.
You may escape the existing algorithms, but they can and do change regularly and chances are that at some point you'll get caught out.