From AmtWiki

Apprentices are individuals who have a formal student-mentor relationship with a Paragon. Apprentices are taught the ways of the Paragon’s class and have a desire to improve their abilities within that class. Players are typically Apprenticed to only a single Paragon at a time. Paragons of a class may themselves be Apprenticed to a Paragon of a different class. The reserved symbol of an Apprentice is a belt favor in the color of their Paragon’s class trimmed in silver.