Belt colors
White belt
The color white signifies innocence, as that of the beginning student who has no previous knowledge of Karate Do.
Yellow belt
The color yellow signifies the earth. The beginning student begins to create a firm foundation in Karate Do technique, just as a seed begins to expand its root system deep in the earth as it begins to grow.
Orange belt
The color orange signifies the changes of Autumn, as the student's mind and body begin to develop and grow as a result of the new Karate Do experience.
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 Karate Do.
Blue belt
The color blue represents the sky. Reminding the student to reach for the heavens and continue their Karate Do journey.
Brown belt
The color brown represents the ripening or maturing process as that of the advanced Karate Do student whose techniques are beginning to mature.
Black belt
The opposite of white signifies maturity and dignity, as that of a senior student of Karate Do, who has learned the basic curriculum of Karate Do and is ready to become a true student of Karate 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.