It doesn't have to be a ribbon. TalkingHands (aka Lisapooh) has a lime green backpack with her name on the back), and I've seen several DISers with lime green mickey heads (laminated paper or plastic), lime green hats/visors, and lime green shirts.
Every time I've seen someone with one on, I've said hello (except for one time, when I noticed the green mickey heads after I noticed the angry facial expressions and unkind words... when you identify yourself as a member of a group, you should always be on your best behavior, lest the whole group will be thought of as uncouth.)