A Green Belt is used by the 6th and 5th Gup students upon the successful completion of an ITF grading
Meaning of Green Belt – Green Belt signifies the plants growth as the Taekwon-Do skills begin to develop.
The belt colour meanings refer either to the stage of development of a student, or to the plant/tree which symbolises the Taekwon-do student’s progression.
The belt colours were not chosen arbitrary. They are in-fact steeped in tradition. The colours of Black, Red and Blue denote the various levels of hierarchy during the Koguryo and Silla Dynasties of ancient Korea. (Source)