As a great part of human population has color blindness (and red-green is a common type of that color blindness, see, for example http://www.asktog.com/basics/firstPrinciples.html) the advanced items in a form should be differentiated from the required items by more than just being green instead of red.
To provide further differentiation, change the advanced item sign to [gear] and also include it in the button. Is this utf8 character available in all fonts possibly used with Moodle?
In the wiki document I have listed some other possibilities, which I have thought less optimal than the gear icon, since the gear icon actually can be thought of as a metaphor of what it means and is thus potentially more easily learnable. It is also used in Mac OS so some users probably know it already.
Also consider highlighting the advanced items as an usability (not accessibility) improvement in the default theme. This will help users spot the appeared items and is likely to make the UI feel more responsive. See: http://moodle.org/mod/forum/discuss.php?d=129719#p568302 (the highlight colour could be a bit stronger though)