Karate belts

Karate belts are a measure of a student/teacher ability. For example, a Brown belt is higher than a Blue Belt.
Usual rank.
All styles of karate range from white to black. There are some theories about how belts came to be.
The "dying" theory.
Some believe when a student graduated a kyu, he/she would dye their current belt the new, desired color. This was often done in Japan during WWII when it was a very poor country. 
The "dirty belt" theory.
This is a theory that demonstrates how a student would, as they trained, dirty their belt overtime. More of a myth than a theory, a student started with a white belt. As sweat, dirt, and other debris occupied not only the belt, but the karate gi, his/her color appeared to get darker and closer to black.