NO BELT: Every student starts at this level. They will be taught basics and will be tested to earn the white belt.
WHITE BELT: The color white signifies innocence, as that of the beginning student who has little or no previous knowledge of Tae Kwon Do.
YELLOW BELT: The color yellow signifies the earth. The beginning student begins to create a firm foundation in Tae Kwon Do technique, just as a seed begins to expand its root system deep in the earth as it begins to grow.
GREEN BELT: The color green represents growth, like that of the green plant as it sprouts out of the ground. The student has built a firm foundation and now begins to grow in the art of Tae Kwon Do.
BLUE BELT: The color blue represents the sky. Reminding the student to reach for the heavens and continue their Tae Kwon Do journey.
PURPLE BELT: The color purple represents the changing sky of dawn, as once again the student undergoes a new change and prepares for the transition to advanced student.
RED BELT: The color of blood signifies danger and is a warning to the student to temper her newly found skills and techniques with control and wisdom.
BROWN BELT: The color brown represents the ripening or maturing process as that of the advanced Tae Kwon Do student whose techniques are beginning to mature.
SENIOR BROWN BELT: This represents a seasoned Brown Belt (with black stripe) who is maturing into a Black Belt.
BLACK BELT: The opposite of white signifies maturity and dignity, as that of a senior student of Tae Kwon Do who has learned the basic curriculum of Tae Kwon Do and is ready to become a true student of Tae Kwon Do. A Brief History of the Martial Arts Belt Systems
A brief history of gup/kyu/dan (kyu is the Japanese equivalent of gup) ranking systems and belts, follows:
Before Jigoro Kano invented Judo, there was no kyu/dan ranking system. Kano invented it when he awarded "shodan" to two of his senior students (Saito and Tomita) in 1883. Even then, there was no external differentiation between yudansha (dan ranks) and mudansha (those who hadn't yet attained dan ranking). Kano apparently began the custom of having his yudansha wear black obis in 1886. These obis weren't the belts karateka and judoka wear today - Kano hadn't invented the judogi (uniform) yet, and his students were still practicing in kimono. They were the wide obi still worn with formal kimono. In 1907, Kano introduced the modern gi and its modern obi, but he still only used white and black.
Karateka in Okinawa didn't use any sort of special uniform at all in the old days. The kyu/dan ranking system, and the modern karategi (modified judogi) were first adopted by Funakoshi in an effort to encourage karate's acceptance by the Japanese. He awarded the first "shodan" ranks given in karate to Tokuda, Otsuka, Akiba, Shimizu, Hirose, Gima, and Kasuya on April 10, 1924. The adoption of the kyu/dan system and the adoption of a standard uniform based on the judogi were 2 of the 4 conditions which the Dai-Nippon Butokukai required before recognizing karate as a "real" martial art. If you look at photographs of Okinawan karateka training in the early part of this century, you'll see that they were training in their everyday clothes, or in their underwear.
Most other arts that have ranking/belt color systems adopted them from the Japanese