Okay, I'm only a year into active duty as my son's pack's awards chairperson and I'm stumped.
In the past, I think the Tigers have gotten service stars on orange backs. Wolves thru Webelos got stars with yellow backs and the number count started over again at Year 1. So a boy with 3 years of Scouting, beginning with Tigers, would have both a star with an orange back and a number 2 star with a yellow back.
However, when I tried to buy more orange backs at the Scout store, it no longer sold them. So I bought the "Insignia Guide: 2002-2004" booklet. It states that yellow backs are